Cipp is right on the HS.
If you join arena with a pet its often bugged. Always spawn a new pet in the buff room.
There is another huge bug with counterspell. Sometimes you can cs a player that is behind a pillar when your pet is at his spot and sometimes your WL needs the line to place the cs. Thats really bad. Should start a topic at WoW support while Im thinking about it... yeah, will do that.
TBH I always summon a new pet in arena but it still bugs sometimes...What about the grimoire of sacrifice and the infernal? They bug randomly I still don't get why....