I think I’ve (accidentally) found a reliable way to reproduce this:

1. Have a card with a checklist
2. From the card’s hamburger menu, select More -> Delete, but don’t touch the (”Delete card?”) confirmation prompt!
3. With the card deletion confirmation prompt still visible, delete the checklist (choose Delete from the checklist title, then confirm the checklist deletion)
4. Now select Delete from the (still lingering) card deletion prompt

Boom! The card (and the board) is now undeletable.