<OP> You're right. It's really unpleasant. I hope and believe that most people wouldn't act like that - and that at least some of them would grant the draw.
It also depends how the position was BEFORE your mouse slip, and what's the relation between the ratings.
If you had a bad position anyway, or you had much less time or your clock (in blitz), or if you're lower-rated by about 100 points or more - I would never grant the draw.
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I don't think you can do take backs here but at other sites I'll definitely grant one for an obvious slip, although I've never been granted one when I've slipped and requested one. I think most are the type to decline when it comes to stuff like that unfortunately
And wait until you get cursed for not accepting a draw.
I love the ones that cant speak English during the game then when you beat them they curse you in perfect context.
To be an honesto abe, there are many players who find it fun to insult their opponents during games. Causing someone to be enraged is a rather entertaining thing, and it can also affect their gameplay. In this case, the man you were playing was not an honorable hombre; he, for some reason, cares about his virtual rating in a website where barely anyone under 1800 plays viable openings or has any tournament experience.
Why should you get a draw? It was your error. If you make a fatal mouse slip just resign, tell your opponent how you screwed up, and ask if you can start another game.