Here’s the scenario:

Opened a card type for editing, removed a card property (which was used in some transitions). Got a warning that the transition (which used the card property) would be removed. Pressed ‘ok’.

Noticed subsequently that the transition which should’ve been removed automatically was still showing as an option, so tried to remove. However, any attempt to select ‘card transitions’ resulted in an error (the generic ‘Mingle found an error it could not fix’).

Adding the card properties I’d removed back into the card type allowed my to get into transitions. I could then delete the problem transition, and remove the card properties again.