1. The opponent is beaten two or three times in a row and asks for rematches after every loss. Finally, on the fourth or fifth time, the opponent wins. The winner asks for a rematch, but the now victorious loser is gone. Unless there's a good reason for the departure, that's not cool.
2. The flipside to this is when the opponent has been beaten a couple of times and asks for a rematch, but the winner declines the offer, the loser then continues to try and play the winner in new games. No means no.
3. A player sets up a timed game. An opponent joins the table and...does nothing. Fish or cut bait.
4. Trash talking. Already covered in another rant.
5. Losing player gains a perpetual check but without any way of making it stick. Nevertheless, he won't draw. Not cool.
6. Ungracious winners or losers. Sort of a corollary to #4, supra, but slightly different.
7. Declarations by a losing player that the winner was somehow lucky. There is no luck in chess besides getting white to open the game. If the winning player beats the losing player when playing black, where is the luck? Moreover, does winning with white owe itself only to luck without a scintilla of skill? Grow up.
8. On this site, losing the connection. It's frustrating.
That's all for now. I'm sure I'll come up with more later. Feel free to add your own.