The description of the bag of holding (DMG, p. 152-153) says:
Placing a bag of holding inside an extradimensional space created by a handy haversack, portable hole, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it to a random location on the Astral Plane. The gate then closes. The gate is one-way only and can't be reopened.
However, the section on alterations to magic in Curse of Strahd (p. 24) says:
No spell—not even wish—allows one to escape from Strahd's domain. Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, […]
What happens if you put a Bag of Holding inside another Bag of Holding (or Handy Haversack, etc.) while in Barovia? Even if the portal fails, what might happen instead?
Best Answer
Item breaks, but nobody leaves because Barovia has already considered that option.
Curse of Strahd explains the "you can't leave" aspect of Barovia:
(emphasis mine)
5e's Bag of Holding includes the warnings:
and
One of the meta-rules of D&D is "specific beats general" - that is, a more-specific rule can override a more general rule. The Barovia rule is more specific than the Bag of Holding's rule - it applies only in one place (where the Bag's rule applies "everywhere"), and it explicitly states that it overrules other, more general rules.
Thus, within Barovia, Bags of Holding cannot spill creatures into the Astral Plane, nor can a Bag of Holding and a Portable Hole create a portal which allows creatures to enter the Astral Plane. They can be destroyed, of course.
It's unclear whether the non-creature contents of an exploding bag would be ejected to the Astral Plane, however. FWIW, this GM would probably allow Strahd to choose, much like he can make himself the target "spells that allow contact with beings from other planes".