I realized lately that i have some pet peeves about chess opponents, maybe someone shares them:
1. idiotic draw offers, especially by juniors (i forgive them, but someone has to teach them that). You don't offer a draw if you're 300 points lower rated and an exchange down, just because you moved your Queen back to the square it just came from. If you're 300 points lower rated, you better have a winning position to offer a draw. Even in a drawn position imo it's on the higher rated player to offer the draw, but i may be too strict on etiquette here.
2. resigning when it's not your move. It interferes with my thinking process and messes up the scoresheet.