Re: WOW- I lost my pet bar!
It is a very old bug, and as annoying as hell. I never had this problem in patch 1.x. The bug I did have then was no pet bar showing up if taming a new pet for first time. That occured from when I first played a hunter, and had no addons.
Ever since 2.0, the bug you mention comes up, it's constant and irritating. Dismount or get dismounted, and bam. Umm, hey Blizzard, not everyone keeps their pet on defensive, and under certain circumstances one should not. Realoading the UI fixes it, but if your in combat, that's suicide. Best one can do in outland at least, is feign, have the pet take hits, then hop on flying mount, get off ground. Fighting mobs without the pet is a big no, unless they're low enough lvl by comparison.
The one that really got me, was when I had to take control of certain mobs to destroy warp gates in netherstorm. Dismiss pet, go to take control, then the old, even 1.x bug I mentioned comes into play. Reload UI, and my temp "pet" is killing me when I come back. Finally had to party, and get someone else to control the mob for me. As to Chess in kara, dito, the very idea bothered me, as I didn't know I'd get a bar for the chess peice.