Not many answers, huh?
Lisa can get very angry only because of her beliefs. And I almost share those with her, so I'll explain how I'd behave if Lisa was a little angry at me:
I'd honestly ask her of the exact reason of her anger, and if she couldn't explain it I'd tell her to calm down, maybe play the sax, until she finds the right words. Because I'm ready to accept the opinions of others, and she'd understand since she has similar traits. And I think she'd then forgive me.

I've needed 15minutes for that text cause I really tried to imagine such a situation. Oh now it's re-submit 

SimpReal
Hm.. can't remember, but I do know Gustavo's post is gone! =(

I'm gonna go with Maléfix on this one for most of it.
People get mad at me approximately.. never. And having a friend as precious as Lisa, I wouldn't even dream of disappointing her. If I'd for some reason ever let her down, there would surely be a good explanation, and one way or another I think she'd understand. Homer's got away with a few things without good explanations, right? ;x
Now... let's forget everything I just said. If she would get really upset with me, I'd of course be sad (whether she understands it or not) to have disappointed her. If there'd be absolutely no way to make things up to her I'd be devastated. Although I can't imagine a situation where I'd "realise" that. o_O I'd keep trying to set things straight in any way I can, while trying not to bother her by doing so. It's difficult to say what I'd do if I gave up, depends on what my life looks like overall at that point. At the moment I don't have anything to cling on to, so I'd probably go depressed and ignore my life for at least a few months. If things were good I'd probably make sure to keep earning my grades at school, do my chores and what not, but still be pretty sad for most part of the days. If things were really good and I also had some other friends that I enjoy talking to, I'd probably be sad at times, but still be able to enjoy daily life.

I realise perfectly that with my endless laziness and ignorance I would be a PITA for Lisa. If we were near to each other, I can easily imagine situation when I disappoint her, even close to the bound of breaking. What would happen afterwards? Okay it would make me very depressed, but, I wouldn't kill myself for sure, I wouldn't savour my depression either. I think I would wait few days and try and talk the problem over, to say sorry if needed. Lisa is a nice person, she is quite able to forget and to forgive, and time cures everything. But if, like Shadow Nait said, she would never forgive you... damn it would be awful. The best way would be to move somewhere away and to try and live without hurting others.

Maybe in several months she would find you and try to make things up... that sounds like a dream and therefore unlikely, but if that happens, don't miss your last chance. :-)

Lisa?? Lisa Simpson? Disappointed in me? The universe would collapse if that happen. Because it never happen. I disappoint nobody who I care for. All people know that. My family. My girlfriends. My friends. Even my former girlfriends.

Well, duh... it would be same as the situation any other important person to me would be disappointed in me. I would feel very upset with myselfe. Same with my girlfriend. If she is for some important reason dissappointed I try everything to make her happy again.

oh that's true of course. But I think I'd mostly do what I already posted, if I wasn't already in a time-space-collapsed dimension...

Okay... I'll indulge in a little bit of hypothetical thinking here...

Assuming Lisa was real, and assuming she was a friend of mine, if she were to be upset at me somehow, I'd pretty much treat it like I do with everyone else. For example, a friend of mine gets mad at me, and I don't know why. I ask him or her what the problem is, and either they tell me and we work things out (usually), or they refuse to speak to me. At which point I just ride it out until they come around, which can be quickly when they realize that the silent treatment goes nowhere in solving problems.

Gary made a good point though - it really depends on why she's mad at you. I've had friends mad at me for a variety of reasons, some of them are my fault, some of them are their fault, some are just plain silly. So it's entirely circumstantial.

Miles
Well, Lisa is just as human as the rest of us. If I recall in one episode last season she was upset at Homer because he was refereeing one of her soccer games, and made a fair call that was in favor of the other team. After this she became upset with him and wouldn't talk to him because he "ruined her life".

My point is, she's not perfect, plus she's eight years old. There's much plausible cause for her to be angry at someone, and for it to be her own fault.

Gazmanafc
Exactly what I mean. Homer was right, she was wrong.

Miles
Okay, so Lisa in your world is always right. I was just taking a practical approach to it, and allowing myself to pretend for a moment that Lisa was a normal person like the rest of my friends.

Actually... if Lisa was real, she would be in my eyes something we could call a "perfect person". Perfection is not very human-like, but she would be such person I belive that there are persons like her out there... or they are very much alike.

And why asking for the reason Lisa could be disappointed in a case... I guess IF she is it, she has a GOOD reason. Humans aren´t moody for no reason.

Well, that is for me, after all, I would never disappoint!

G.H.
You're still not reading my posts, I didn't ignore anything you said, I didn't attack your approach either. I did say that I'd have a hard time imagining the old Lisa getting really mad at someone for a stupid reason, yes. If you think about it, it's not really difficult for someone to live by such a standard unless the person is selfish, ignoring other people's point of view. And the more classic Lisa doesn't disregard other people, the soccer episode was ridiculus... and probably other new episodes, only watched few of em since I don't find it very entertaining anymore.
But... I'm sorry for being a bit pushy (sorry to everyone really), I find myself being an annoying know-it-all or the likes recently. I'll try to come on less ill meaning or offensive in my posts, I'm kind of slow with finding the right words though, I get way too picky about my choice of words unless it's more "on impulse". >_<

I would be deeply upset with myself if i did something to hurt Lisa (as would I anyone I care about).

I would have to write to her in a letter how sorry I am and how disapointing her has made me disappointed in myself. And how I would be so sad to have damaged a friendship.

Hell I would write her a poen.... and I am the worlds worst poet, hopefully the effort in trying hard would say to her that I care and that I would do something so silly to put a smile on her face

Miles
It's okay, if anyone comes off as less friendly or more stuck-up than they really are in their posts, it's definitely me. I'm just saying here that I don't view Lisa as any more perfect than the rest of us. Maybe she's mature beyond her years, but even mature people have their faults. I could see Lisa getting upset at someone for a trivial reason - the difference though between the mature and the immature, is that the mature are able to see when a problem is trivial, and make what reconciliations are needed.

I think it's safe to assume that even if Lisa were to get mad at you for reasons that weren't your fault, she wouldn't take long to come around.

CargoOfDarkness
That was a fu**ing hell proud bravado, you know, fella. I don't believe you.
Please, don't forget she's not like other girls, that you probably used to know. And the cliche in the relationships is not suitable to her, don't be mistaken here! I'm trying hardly to believe that... I'm even not counting her as a genuine girl (or woman, for those who's interested). At least not as possible girlfriend or so. I know it's strange approach, but it's my point of view and I believe it the only right way to do. I don't care too much about the other's feelings towards her if it belongs to... eh, love or lust, well, call it as you want.

I believe she's not even a girl (hell, not a hidden boy too! ), such a sexless intellectual substance. And from such roots I'm going further in my reasoning.
---

Back on theme.
If Lisa would be dissapointed in me hardly, if she cried at me, goin' hysterics, I would be shocked of course. And very ashamed... well, you know all this little nasty things that assotiated with being an "adult". I know, she's a smart girl and can understand almost everything. But I'm worry about her uncontrolled reactions too. She's quite hot girl at times and can tell you right in the face everything, that she thinks about you. She's a young perfectionist with a hardly changeable opinion (if it's not her mum... or McCartneys in the flower garden on the roof )

And do not forget that no one of us has meet her at least in a single time. When she has seen us, the adult men... when she has learnt that we are melting with her, she probably will act not too graceful for us. I think we should remeber it.

CargoOfDarkness
This sounds more like "I don't care about anyone who dares to be disappointed in me".

The topic title is "what to do if that has happened", not "would Lisa admire me to the degree of ignoring every stupid thing I'm doing". So, "Lisa would not be disappointed" is not an answer... it only shows that you don't have any reserve plans in the case it would still happen.