Hmmm... interesting point, that. I'd not thought of it, since the UI isn't showing a combat status. And, I've seen it happen in one area, say Grizzly Hills, and be persistent through departing the area for Dalaran, and have it remain for quite some time. I may well be wrong, but I thought combat was cleared when zoning.
Incidentally, I am not 100% certain of this, but if what passes for my memory serves, the issue with the skills page may have coincided with this one. I'll be sure to take better notes when/if it happens again. That one has only happened a few times, so I haven't really paid much attention to the HUD.
Neither of these things are really issues, mind you, just things that make my feeble brain itch at times...
Nothing a reload doesn't fix. With all the work that has been put forth into getting the blank frames thing ironed out, these are small potatoes at best!