Ran into a group of three blue monsters (dervish type). Both of my friends could see all three, but I could only see two. I could still damage the invisible mob, if I was lucky enough to be facing the right direction. I could not click on it, so I could only attack by holding shift and guessing. Thankfully, I could see one friends monitor and was able to kill it by looking at her screen.
Strange event, but definitely a bug of some kind or other. Thanks.