It can be things you don't even think of. For years, I flew over to see my then-boyfriend. When asked by Immigration, I just said I was over on vacation.
Technically, it turns out, that was a lie and could've gotten me banned. I never realized until years later (it WAS vacation, darn it). While I was over for Christmas one year, I finished a job for my US employer and emailed it to them. Turns out, totally illegal.
So, it's harder to be legal than it looks, even with the best will in the world.