I remember taking Spanish in high school. I had already taken several years of another language in elementary school and I spent the first six months of classes accidentally switching over to the other language in the middle of a Spanish question. It turns out that I’m not the only one.
While filming The Frighteners, Michael J. Fox would often forget he wasn’t still making Back To The Future, and yell out for Doc! It happened so many times that they actually caught it on film and Peter Jackson (who directed The Frighteners) commented on it in interviews.