atom:id field of the Hot Questions feed uses the URL of a question, including the slugs which can change over time as it uses words from the question's title. With the current scheme,
<id> changes when the title is edited. May I suggest changing it to use the canonical URL instead of ponies?
I went through this unofficial list of feeds and the Hot Questions seems to be the only one not using a canonical URL.
As a reference, RFC 4287 says this:
When an Atom Document is relocated, migrated, syndicated, republished, exported, or imported, the content of its atom:id element MUST NOT change.