I've had this happen sporadically--perhaps twice total, in any given series of 50 battles.

I get this a lot with the champion Csilla, who casts Mass Curse at the beginning of the battle. If she gets the first turn, this always happens; if someone else goes before her, it doesn't. It has come up in at least two separate games for me.

Have yet to see it with anyone else but can imagine it could easily happen to other units with similar abilities.

You get this all the time if you play Pariden (or any Amarian race) due to their blood trait, or if you are fighting them. It's not 100% of the time, but it happens way too often.