We've all seen it or had it happen. Basically, it's a visual glitch which causes the player, while on the frontbar, to be holding invisible weapons and standing on top of the backbar weapon on the ground. It is at least a year old bug.
There seems to be no exact prerequisite for it to occur, but seems to happen on my sword/bow and dagger/bow characters and not staff using characters. Sometimes going through a door can trigger it, sometimes changing zones, sometimes just out of nowhere in or out of combat.
PC NA but I've heard extensively this occurs on all platforms. Other users report that it has no effect on the functionality of the invisible weapons, but for ne it seems to make things slightly more difficult. That could be solely a mental effect though and not game physics.
Longtimer since beta, the usual. 26 CP toons. ~1700cp on main account, 1000cp on 2nd account. Endgame-ish lol. Most Vets / some HM's cleared.