I'd say it's when you start off well in a game but you know because how well you're doing that soon the other team is going to be loaded up with armor/nailboards/bombs/molotovs.
Smokebomb + shiv. I just hate how helpless you are xD
Also molotovs or bombs when I am not even close. One time I died because of the ground shaking impact of a bomb and the guy next to me died as well.