Have a slot which is haunted with a sigil via the haunter sigil.
Have that slot be opposed by a creature with the Sentry Sigil.
Place a 1 health card into the haunted slot
The creature placed into the haunter slot will die to sentry, but the haunted slot will still try to give sigils to it after it no longer exists, and the game will lock up.
Steps to recreate:
The creature placed into the haunter slot will die to sentry, but the haunted slot will still try to give sigils to it after it no longer exists, and the game will lock up.