I wouldn't. I never progressed beyond chess clubs in public schools and I certainly remember people making illegal moves in tournaments. Like that's why they make you both record all the moves. Because people make mistakes. Though, honestly, I remember more notation errors than play errors.

Accidentally moving into check is probably the most common illegal move. Castling though check is surprisingly common, too. Actually moving a piece incorrectly is fairly rare, though. I remember one tournament where one of the matches ended in a DQ because one of the players had two white bishops.

Could one have two white bishops after promoting a pawn?

Promoting to anything other than a queen is rare, and I expect the next most common is to a knight. Promoting to a bishop, while possible, is going to be extremely rare.

Yes it's theoretically possible to have two light-squared bishops due to promotions but so exceedingly rare that I think most professional chess players will go their whole career without ever seeing that happen.

Outside of playing a game for piece value? No, not really.

In this case, of course, someone moved their bishop from black to white and their opponent didn't catch it until awhile later.