I dealt with the most bizarre kind of griefing on a public KF server last night. I was playing Mountain Pass with one other guy (a level 4 medic). We were on wave 3, and doing alright. Two level 6 players showed up, both with the new robot skin. At first, it was kind of cool. They fired off more or less every voice command, all of which are robot gibberish. They also did some pretty good killing. One was an L6 berserker and the other was an L6 sharpie. Things started to get odd when at the end of a wave, the last 3 zeds were all Scrakes. The medic and I were on one side of the mountain with the scrakes, and unbeknownst to us, the robots were already on the other side by the trader. We asked them for help and they said "no" in the robot voice half a dozen times. Fortunately, the medic and I were able pick off all 3 scrakes without dying by shooting at them while going up the mountain, backwards. I don't recommend that tactic.
Around wave 6, they locked themselves in one of the small buildings and refused to come out. Since there were now 4 of us on the server, the waves were proportionally bigger. This left me and the medic to fight a LOT of zeds. As an L4 commando, I'm lousy at killing big zeds, and an L4 medic is not much better. We managed to get through the entire, mostly by running and some deft healing by the medic. The robots did not exit the building for the entire wave. That is the strangest form of griefing I have ever encountered. Pubbing is often a bizarre experience.