If he/she's a friend and the game is just a friendly game, just let it go. If it's just a normal game however, then I think the principle goes: He/she has tried to gain time by being less than careful and hasty about moving the piece, took a risk and simply failed this time. It's no different than a risky aggressive move that needs to be properly punished if possible. So just go ahead and take advantage of the "blunder."
Mouse Slip - how to react?

I can only tell you about myself:
1. If I make a critical mouse-slip, I resign without comment.
2. If my opponent makes what I suspect to be a mouse-slip, I take that piece, quick as a flash. Any moaning = no replay.
Chess is not a "second chance" game, all moves are final.

+1
I'm a big fan of games that end.
Fretting means the game's still going on for you. Get over it and move along with your chess life.

These earlier discussions are a reminder of what a minefield allowing takeback would be ...
honor-vs-pragmatism-chesscom-and-precolonial-ideology

Every move I make is a mouse slip!!!! For real... the mouse slips I hate most are the ones my opponant makes in a close and fun game... then I am sad that the mistake was made, but do understand that all moves are final. If you can learn to live with disappointments... then you wont have diappointments.

computer mice slip, whether being "hastey" or not. chesshere.com used to have a "confirm move" check box where you had to confirm every move. if I ran a site, I would have enable an ability like this. but for this site with no such feature, take the piece and dont fret about it.

Chess traditionally is a "touch move" game... If a piece is clicked and reclicked into position that should be the end of it, opponent to move, no complaining.

If I'm sure that it's a mouse slip then I offer a draw.
For example, an opponent was going to take my rook with his rook, but it stopped one square short where I could capture it for nothing. It was an obvious accident so I offered a draw.
If it was because he was playing too fast in the opening, then I may or may not depending on how I'm feeling. For example, I've had some games as black that have gone 1 e4 d5 2Bc4 dxc4. Usually I've offered a draw and we started again.
And by the way Postafi, I'm not a "true gentleman"!.

Gosh Postafi, you are awake late.
I guess a 2nd question would be, would you ask for a draw if it was you who had made the mistake? I'd be too embarrassed.
And a 3rd question. When did the forum start, and what could be the oldest to be revived? Not that I would do that. Oh no, not me.

If I mouse slip, then I just play on and see what happens.
Sometimes I'm like "hey, that's better than the move I intended to play!"

If I mouse slip, then I just play on and see what happens.
Sometimes I'm like "hey, that's better than the move I intended to play!"

I guess I'm just different, starting with who the frick cares if it's an old thread? why does that ever bother people - is it THAT important that you tell everyone that you were "there first"? Were you born first and so why do I care?
There are at least 20 reasons why my mouse might slip. Sometimes it happens right when I'm in a far superior position. I don't make excuses BUT I think it's very important to let my opponent have the information in order to do what they think is right. In almost every case the opponent thinks it right to just ignore it. I have no problem with that. However, there are a great many games when I'd really like to know what happened. If I'm getting destroyed and my opponent does exactly what someone here described: moved the rook to a spot one square short when it's obvious they meant to take a piece ... no way to I continue. I'll draw or just resign if they don't say anything.
Again, so many ways a mouse slip can occur that is completely unrelated to touch move, etiquette, or any of the other "gentlemanly rules" that don't involve electronic equipment.

The proper etiquette for a mouse slip is to take it on the chin, and continue playing down material. Or resign.
Accusing your opponent of taking advantage of your mouse slip is terrible etiquette. And it sounds to me like your opponent missed this day in etiquette school.
Net time an opponent does this, turn off chat, And/or block him.
I come to Chess.com to escape the Drama of daily life. Not to pursue it..
My question to the greater chess community is, what is the proper etiquette if your opponent has a "mouse slip". I recently played a game where my opponent made a bad move and claimed he had a mouse slip. I would have let it pass but he accused me of taking advantage of him. I considered his behavior petty and ungentlemanly but it begged a bigger question. What is the proper thing to do? There must be a rule in regular chess if a player drops his piece, etc.
Yours truely, Hardboiledeggs