Version: 0.76 (maybe this was fixed in 0.77)

Conditions:

• Tactical Battle.
• Sovereign casting Summon Ice Elemental (multi-turn spell).
• Bad guy (Air elemental I think) cast Titan's Breath and knocked down my sovereign mid cast.
• The casting resolution logic seemed to happen prior to resolving whether my sovereign could get up.
• Sovereign stayed in prone position.
• Could not cast spell (see the message in the image)...
• but could also not cancel casting of spell.

Workaround:

• Auto-play battle button still worked. If this wasn't available, I probably would have had to alt-tab-kill the game to get out of it.

See image below: