just my opinion, but as I way too often get killed by this, I absolutely hate and already thought about quitting for good is:
invisible hostiles or the ones suddenly dropping from/through the ceiling when you enter a room
absolute impossibility to do any change on resupply because it will get you killed more often than not
and, last but not least, the enemy spawn in the recently captured/cleaned area.
I´m not even talking about when the capture leads to "secure" without counter-attack, but when you fended off every single hostile and you just re-supplied and put your gas mask on there he pops up out of thin air - the mysterious suicide bomber or shotgun specialist from behind and leaves you dead for the entire round.
While I cannot change it I always leave the match instantly because it drives me mad...
Anyone feeling the same? Having other hate issues? Comment below, please