Overpull

A big mistake I see a lot of warlocks do is overpulling with whatever demon they're using. VW's good for two (maybe three), but the succubus is only good for one at a time. But young warlocks don't know this, so they send their demon into a pack of mobs, start up with their DOT's and suddenly find themselves in the middle of melee because the demon can't hold aggro.