Stack Overflow users are automatically blocked from asking questions when they have had too many downvoted or deleted questions. (The details of the formula are secret.)
When another site attempts to migrate a question to SO, if the asker is blocked there, the migration is instantly rejected and the question stays closed on the source site. This shows up in the question history¹, but you have to go looking for it.
Should moderators point out what is happening? This exposes the information that the asker is question-blocked, which is normally not public. Or should moderators remain discreet (which makes it look like they didn't attempt to migrate the question, and may leave the asker unaware)?
¹ “Post Closed by …” immediately followed by “Post Locked by Community” (as in a real migration) immediately followed by “Post Unlocked by Community” (as in a real migration rejection due to closure on the target site).