Some high end Velious encounters (for example, Vulak Aerr) summoned via a spell and not the ability:
http://lucy.allakhazam.com/spell.htm...80&source=Live
Mobs with this spell could summon much more rapidly and at 100% HP. This was done to avoid pre-debuffing and aggro building before engaging a raid mob.
Edit: I also remember mobs with this spell (which I think was just all the Lords/Ladies, Aaryonar, LTK, and Dozekar) would spam it. Tanks would constantly have to reposition every 5 seconds, and you would get summoned as soon as you entered their aggro range.
As the game progressed summoning wasn't very much dependent on level. I remember some really low level (20-30?) mobs in Luclin summoning. Alternately, the level 68 mobs at the "tables" in PoFire wouldn't summon, I used to solo them all day at 65 for some phat AA exp.