The biggest mistake I made was watching the stick movements whilst being shown how to do my first aileron roll. Head down, no outside visual reference. Fine first time, but then I said 'Do that again, so I can watch the stick one more time"......
I was sick thereafter during almost all of my aerobatic flights. I'd grab the bag, be sick, then carry on. It must have at least been partly psychosematic as occasionally, even now the smell of aviation fuel can trigger a vague sense of nausea. Didn't stop me enjoying the flights and wouldn't stop me now, but very difficult to get rid of!