[blahpro]

My guess: many CDNs allow you to exclude the querystring from the cache key, so it's possible that one person requested the URL with ?torrent in the querystring (which causes S3 to serve a .torrent response) and that the request hit a cold cache. The response with type application/x-bittorrent was then cached under the querystring-less cache key, causing it to be served to anyone else hitting that edge node with the path /widgets/tweet_button.html.

Again: this is just my guess.

[gazarsgo]

I thought Twitter was all private DC, did platform previously point to S3?

[blahpro]

platform.twitter.com is currently CNAMEd to EdgeCast CDN. It looks like the CDN is sitting in front of Amazon S3; http://platform.twitter.com/blahblahblah gives an S3-like 403 response.

[StavrosK]

This is my exact guess as well. I would be surprised if it turned out to be something else.