I used to get pissed and think it was a hack when people just knew where I was, over time I learned about the EOD(?) bot and the T-UGS by playin my least favorite class, the sniper. So I was completely wrong in those cases, but that was early in the game.
The other was the BS IRNV in its original form, man that was such an example of a completely inept game design decision and a QA that had no balls or holds not clout to say "This is a fucking dumb idea as is...."