I have seen this at piano recitals a lot. Generally, the person it happens to gets frustrated, starts over, and then it goes okay. Then there are the people who make little mistakes in the middle and get upset about it. Then, there are people like me who cover up their mistakes by improvising something (not always a good idea!). I remember playing at my first recital (it was a Bartok piece) and i made one little mistake in the middle, but no one noticed -- that's because I didn't show it. At my most recent recital (this summer) I played a Schubert impromptu -- it was literally an impromptu. I sat down, everything was fine until I hit a note that did not fit anywhere in the general flow of the music (key of a-flat), so I had to create a chromatic passage down to the original theme (not good!). This was my way of covering up a "faux pas" of sorts, but it is in itself a faux pas. In a bit of advice, keep going! It's great to practice in front of audiences before, and try out different pianos if you have the chance.