This evening, I somehow ended up with two pets. At one point, I realized I had a pet by me, but no pet bar. I tried to dismiss, but it said I had no pet. I called another, and voila... I had two pets!
One was my normal pet that I could control. The other was like a hunter pet below level 10 who just did what he thought was best. In one shot, you can see the automated one attacking Kael Thas while my controllable pet is on passive.
As soon as I hearthed, the second pet disappeared. But hey, interesting bug...