I've just encountered the most challenging bug in my career (as an ERA bug fixer).
Edit: Just a second thought, can this be related to the change in ERA 3.9.1x?
And yes. This didn't happen in ERA 3.9.13.

Enchanters can cast massive spells for the wrong side
For example, casting Haste for their enemies.

To reproduce:
- Initiate a battle with anything, like a neutral stack
- Cast Hypnotize on any of the stacks (must make sure the stack id is the same with Enchanters encountered in the next battle)
- Win the battle
- Initiate a battle with an Enchanters stack (or fight against an Enchanter dwelling)
- Defend until the hostile Enchanters cast
- You will find that they cast Massive Friendly spells on your troops

Video and testing map:
Download

Things I have tested:
- Reset Hypnotize spell info from stack structure (stackStructure->0x288 = 0), it doesn't seem to have any effect
- Checked out sides data in the Enchanter function (0x447D00), everything seems to be correct
- Used only raw ERA without Game Bug Fixes Extended, didn't help
- The bug doesn't exist in Complete/Hota (but I haven't checked them without HD, the game didn't launch)
- The bug doesn't exist in WoG 3.58f without HD mod