Sadly, this has happened to me, and I have seen it happen.
The most recent one I can remember was in my friend's HellHound RP, where they would hold Arena fights for entertainment.
In the middle of a 1 vs 1 fight, one person had to go.
About half the group started being like "Oh, NOW you have to go. I get it." and other things of the like.
Me and my friend found it rather rude, and ended up having a small talk with the members after the person left.
No one listened, heck, someone left the group.
It is VERY rude, and I find it annoying.
I've seen it happen in the middle of ANY RP, not just fights.
I can understand under this:
End of an optional fight, their charrie is loosing and/or dieing
They suddenly pop offline, and you see them in your friend list under another character.
If that happens, I give partial rights to snap at them, due to the fact that if they WILLINGLY fought, they knew their charrie may die.
But otherwise, I normally get on my members about being rude like that.