what do you do when youre opponent mouse-slips?

What would you do if your opponent over the board in a tournament game touches a piece ... then sees he/she has not any good moves with that piece ? You wait until that piece is moved ofcourse and there is no 'changing' your 'mind' ... what would you change it for ?

At my level it's impossible to tell if it's a mouse slip or a genuine blunder

One time it happened I went on as usual but because I wasn't expecting it and premoved (it was the opening) I lost my chance to win a queen for free