Defusing mines on escort: "Hey look at this funny guy over there. What is he doing? Tieing his shoes? Booooooom."
You know that your team will lose in five minutes, you know why your team will lose in five minutes, you are the only one who seems to know how to prevent this from happening, but nobody listens to you. AKA The Cassandra Principle.
When you are being rushed by vehicles, and everyone is running around, being a crazy-ass engi, not helpint. When the commander doesn't communicate, like telling the team "hey guys, build vehicles now, we have a shitload of cash and must attack!" When everyone plays singleplayer, not squading-up, pushing each others' tanks into the firing line.
I see this too often on flag-based maps. "YUH! LET'S CAMP AND THEY NUT KEEL UZ!" There's a bleedflag, dumbass!
When being pushed onto enemy mines by friendly tanks. "Hey stop driving, the second seat is taken and....MOTHER F-"
Idiots who know how to play and dont while being a troll or guys who do not know how to play and think their all 1337.
Getting mowed down by HMGs/rifles over and over again. Less so nowadays, but I used to hate that in 2.12. At least I had my bitchin' SMG2 to counter.