Unlike the Unreal1 monsters (UTcoop let you play as the monsters) there were certain things coop allowed that could make games seriously unfair. For example, all the SkaarjTroopers have an energy shield on one arm that lessens the impact of incoming fire to enable them to survive...
...whereas in UTcoop, if you played as a SkaarjTrooper your shield could survive multiple redeemer hits, instagibs, etc. You only became vulnerable if you put it down.
So, for those that played UTcoop ever... what was your favorite exploit you would do? Something that kind of evil.
I liked to disappear, sneak up right behind someone, and then missile them to death. Turns out in coop you didn't need to reappear to shoot your missiles.