For clarification, an impossible move being a fail is annoying (Be1-e2.) Moving into check could be a fail as it represents a chess error, not a mouse-typo.
Maybe, but you can't make such a move in real-life, so a failure would represent another difference from reality. It is likely harder to evaluate the type of illegal move anyway. How do you determine what is a mouse slip and what isn't.
I could go either way on this, but I have had it (mouse slip) happen to me (and lost points). I was my fault though as I was getting lazy all slouched down and such.