A set of four coffins popped litterally out of nowhere (in just one frame, without the animation) right at the entrance of the last boss battle in Sunken Temple of Qarn (Hard Mode). I could walk through it but obviously couldn't see through it, and it was difficult to see when "real" coffins spawned at the exact same place. I asked my teammates and they couldn't see it, so it's client-sided.
I am playing on PS3, and I don't think this could possibly be caused by my PS3 system.
I don't know how many times this happens, it just happened to me on the only run I've made.