Situation:

Campaign, playing as Illusionist, near the beginning of a fight against Deomir. My free Priest of Fire was in slot 3. Spells cast were:

Turn 1: Merfolk Elder (slot 1) / Faerie Apprentice (slot 1)

Turn 2: Fire Drake (slot 2) / Flame Wave

Bug:

When my turn started, I couldn't click any cards or buttons. (They would light up when I put the cursor over them, but clicking did nothing.) I had to close the game manually.

When I restarted, that campaign character was gone -- he's vanished from my list of campaigns. Very scary!