During the Crota fight there are multiple bugs. I have wizards spawning before the first swordbearer. Swordbearers despawning and respawning behind the temple walls and Crota himself is slightly bugged, his shields drop to 0, he takes a knee and as soon as our sword bearer pops up his shield automatically recharges and that's a wrap.
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If you kill the knights on the towers it spawns a wizard on hard
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If you have found a bug you should post it in the feed back forum. That would help Bungie deal with it.