So my group recently defeated a mummy lord and learned a bit too late that they had to find his heart and destroy it. The heart "got away" and they spent a few sessions tracking it down again and having a pretty tough fight with the new guardian for the heart. For reasons I won't get into here, the guardian was holding the heart in a kind of stasis that didn't break until the guardian was defeated, so the mummy lord was on the verge, but not quite, about to reform.
Because the fight was so tough, the group decided to punt on dealing with the heart and stuff it into a bag of holding, figuring that even if the mummy lord reformed, he couldn't just waltz out.
I did decide that the mummy lord could reform in the bag of holding, fwiw, based on the monster description. Because the party decided to wait awhile before dealing with him again, I also decided he had enough time to refresh his spell list and use Planar Ally to ask for a patron for some assistance in escaping the bag, which the patron provided.
Was this a legitimate call on my part? The party thought it was pretty hilarious for what it's worth, but I don't want to have cheated them for what was a pretty clever idea.