Once upon a time pets did exactly what you told them to, run up to whatever you told them to and hit it, predictable. At this point the stupid rested squarely on the shoulders of the hunter or warlock not to manuever their pets behind their targets. Then things change and pets automatically put themselves behind their targets and with that the stupid shifts to Blizzard's implementation.
A fix for a minor end game pet management inconvenience manifested itself in a very unfortunate way for solo hunters and warlocks. You send in your pet and it might choose to run off with the mob and try to pull as many others as it can. There are many theories as to why this happens, but the one sure thing is that it is related to the pet positioning changes. Obviously the change wasn't "ready", but was released anyway and hasn't been resolved for months.
The result is that the very first impressions any new hunter or warlock has of their new class/pet is of a bug, total immersion killer. I have a hunter at 80 that pretty much exclusively groups and no longer solos so I had all but forgotten about this bug, but I've recently started leveling my warlock again and now it is fresh in my mind once again. Hopefully this will get some attention for Cataclysm, if not sooner, since there is so much emphasis being placed on improving the leveling experience for new characters.
There is a workaround to avoid an untimely death, tell your pet to follow then shortly after that send it to attack again. You learn to catch the bug early after several repetitions so your pet ends up more or less where you want it. On the bright side this bug does teach new hunters and warlocks to be more mindful of their pets.