Last night while on my stam sorc i have ran into this bug:
I was using dark deal and got interrupted, then i swapped to my second bar to use vigor, but it was greyed out like it was also interrupted(even though its instant cast).
Now I have never seen this happen before, granted, i dont often look at my skillbars , so its possible it may have happened in the past.
My best guess would be that due to the lag while on client side i was able to weapon swap, the server didnt catch up, and simply was getting data that the skill in that spot was disabled?
I should also add that dark deal and vigor sit in the same spot(5) on my bars, thats why im thinking this couldve happened.
I was able to use all other abilities on that bar, but by the time i streaked away, i was able to use vigor again.