Not really gameplay I suppose, but…
Definitely the strangest bug so far - on Underground, I once spawned as some sort of ghost. Everything seemed normal until I realised none of my bullets were causing damage and I was being completely ignored, thus making me feel like I had actually gone crazy! I then realised that I could still revive, which I think confused my team-mates as much as me 
I soon got a message telling me I would be kicked for idling so I think I then respawned a couple of times and everything was back to normal. Er good luck tracking that one down!
Another regular issue I’m getting is a type of lag. Even when my ping is low and consistent, I often find myself warping about, leaving me staring at a wall, getting killed without seeing it, or popping out from behind a wall, going back and popping out again. Is this server lag or some sort of bug?
One more thing: More of a feedback item, but it took me a long time to realise vsay existed in Dirty Bomb - mostly because it doesn’t feature in the key bind settings