Our goblin druid posed a question today which made our DM bluescreen:
"If I turned into a giant toad, swallowed a Medium-sized creature whole, and then turned back, what happens?"
The goblin druid is a Small creature, and the giant toad is a Large creature. The creature being swallowed is Medium-sized.
This is essentially the inverse of this question, though not quite a duplicate because it's the eating creature doing the shapeshifting rather than the eaten one. Closely related to this question, with the primary difference being wildshape by a willing druid vs polymorph of an unwilling monster.