Irrevocable Actions?

[Fantomen]

Sending messages, items and gems to other players.

Cancelling recruits remaining from the previous turn resets resources, for example a resource 21 indie commander in a resource 12 province, if you bought him last turn and accidentally cancel him you'll have to restart or wait 2 turns again. I picked the indie commander example because that's the one I can think of having real significance.

Removing troops from a commander that for some reason has had his leadership decreased below the number of troops he commands, the troops cannot be moved back to him once removed. Heroic stupidity and a few afflictions does this, not sure exactly what all the possible situations are.

Shapechange into a form without the shapechange command. A few mods use this, like insectoids where the hive queen cannot be made mobile after changing into her immobile form.

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Not something that falls under this category, but in the same vein perhaps: Prophets cannot be renamed, you must rename your prophet before making him your prophet. Makes sense thematically, but I've been really frustrated by it in RP games.