Too many random encounters. You know that feeling when you just want to get through an area towards the next major mission and every 2 steps, some stupid enemy challenges you. Especially when it's a Zubat.

Timed missions. When the countdown reaches zero, I blow up.

Also, the first time I fought a Lambent Berseker in Gears of War 3 on Hard. Stupid muther keeps on chasing me so I had to dodge and roll half the time instead of shooting, which is pretty easy until she starts leaking imulsion and kills me on contact.