Mine is when people don't ping the enemy (or spike) especially when they're dead and spectating and double especially when the spike has been smoked off.

What's yours that people can improve on?

(I wanna add, because it infuriates me the more I think of it - for the love of God ping the spike if it's been smoked off so I can shoot the enemy through the smoke. The amount of times my 4 dead teammates sit back and watch me do it myself where I have to pull up the whole ass map to ping it myself when they can ping and help win the round. Then they have the nerve to get mad at me >:o)