I figured I'd post this here as it only happens to me in PvP:
Sometimes (usually a teleport skill is involved aka Rift Walk) I'll lose sync with the server. The client thinks I'm somewhere different than the server does. I'll run around like normal, but all abilities show 'out of range' or have line of sight issues. Other players will see me in a different spot.
The only fix is to either die/respawn, or exit out of Rift and quickly restart the client, hoping you don't get the 15 minute debuff. This happens to me about once every other week or so, anyone else?