WowAce

It seems to be a fairly common occurrence that the Quartz swing timer will freeze in place with a partially filled swing bar and then never reset or recover until /reload. Seems to be a variety of things that might trip it up like changing targets at a certain time or something, it just happened to me while casting Slam a little late after an auto-attack and then the mob I was fighting "blinking" behind me.

The timer stayed stuck even after combat ended (without typing /reload), but interestingly I pulled another mob, the timer was stuck until the mob was almost dead and then it also blinked behind me and this time it unstuck the timer on it's own, but that was a sizeable amount of combat time with no swing timer. Mobs you can test this specific instance on are called Unbound Ethereal in Blade's Edge Mountains but I bet if you just run around fighting whatever in PvE / PvP swapping targets often and using some Slams here and there you'll see it bug out quite often.