For me:
1. Please do not use a frame pump that just kind of velcros to the clip or carelessly clips in. It needs to be really secure or in a bib pocket.
2. If you don't routinely (like every week) ride in a paceline of some kind, wait till you're off the back from your turn to drink/eat.
3. No tri/tt bikes in a peloton while in the aero bars.
4. If you're on front for a downhill, suffer it down then pull off just before the bottom so the next person has the energy to keep it smooth at the start going uphill. Don't force the folks behind you to squeeeeel their brakes. I won't ride with a group that has the leader soft pedal downhill.
5. If you're going uphill, don't surge over the top creating a bungee cord effect of folks having to hurt to latch on over the top.
6. Cough/spit into your own kit or use the opportunity of pulling off the front to the left to do so to the left. Or off the back.
7. The next "wannabe pro" I see lob a bidon at a road sign or onto the roadside at a fondo or sportive I'm going to get your bib # and make sure the organizing group bans you from their fondo series.
8. Don't surge.