In Hell, Act 3 I encountered the following bugged Elite and Champion packs packs:

Two separate occasions - Plague Herald Elite with Horde, spawning alone with no pack (Fields of Slaughter and Arreat Crater Level 1)

Single occasion - Blood Clan Occultist Elite with invulnerable minions, spawning alone with no pack (Trebuchet)

Single occasion - Blood Clan Occultist Champion spawning only one champion (Trebuchet).
Edited by Hellpyre#1417 on 5/30/2012 2:00 PM PDT