The turrets that get shoved down in the Annex battles in chapter 4 turn into untargetable, invincible Weiss, making Weiss-Roche impossible to beat (at least on Hard) without additional mods. Also, the Reno fight in the church simply does not progress when you kill the first three enemies. Also, I only just now, after posting this comment, remembered there's an actual bug report option.
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after it reaches phase 3 he's just invincible and doesn't attack