When I discover that I made a mistake, there are two things I do:

A: If the glue hasn't cured yet, I work as fast as I can to undo what ever just happend, fix and re-aply

B: if it's too late for that, I'll sit back, scratch my head in disbelief, think about what i did wrong, then clean up my workspace and come back the next day. If I make a mistake that cost me more than 10 minutes, I get too frustrated to continue building. I'm pretty good about not getting too down on myself/throwing tantrums/cussing/using retail therapy etc...which i guess is a good thing!