For me, it was BF Bad Company (2?) on certain places like the rock on Par for the Course where people could shoot out from under the rock but you could not shoot them.. or the other map on Bad Company where one could jump inside the corner of that building on the Oriental map and shoot out but could not be shot. And other places where one could go under the map and shoot up at people capping a flag and the person under the map could not be killed.
The second worst bug was when AT mines disappeared after death (BF 3) but it was eventually fixed.
The following are in no particular order:
BF V: Go to revive downed teammate (standing on top of their body) and either swap weapons or reload your own weapon instead
Spawn on dead squadmate (numerous titles)
Skip reload animation for quick reload (BF 3?)
Hit teammate in face with glowing dart so they can't see for 2 minutes (BF 3)
Spawn on the end of a Cavalry lance (BF 1)
Try to spawn on flag, instead spawn at home base (BF 3)
BF 1: Flying in a plane when round ended caused one to fly out of bounds and get extra death.
BF V: SL can be stolen when order has already been given (this is left over bug from BF 1)
BF V: SL cannot mark all objectives.. seems like only the central flags can be marked.
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