You can reproduce in the YQL console.
Try reloading same site. Usually fails on third or fourth try.
Most likely it's tumblr that is rate limiting YQL.
From the V1 docs (http://www.tumblr.com/docs/en/api/v1): Requests are rate-limited to one every 10 seconds.
You could also write a YQL table that uses tubmlrs v2 API (http://www.tumblr.com/docs/en/api/v2) if you're so inclined.
Thanks for the note.
Could be. No, I'm not inclined to write the table.
Would think someone from Yahoo or Tumblr would since the marriage.
PS Why did it take two weeks to get a reply?