Update January 27 2015: Tweetdeck has fixed this bug in their client, thanks! Other clients still have it.
There have been reports of blocked users showing up in some people's mentions on
Tweetdeck, Twitter for Mac, Tweetbot, and Echofon. This is most
likely because of two factors:
It's likely that many streaming clients make a single call to /blocks/ids on startup and do not implement cursoring for that call. In those clients, users that block more than 5000 accounts may receive unwanted mentions from accounts they block. The response from /blocks/ids happens to be ordered so that the first 5000 accounts returned are the most recently blocked, so the most recently blocked accounts will be filtered and accounts blocked earlier won't be.
Clients that rely on the streaming API to deliver mentions should make sure to implement cursoring on /blocks/ids to get the entire list.
Note that there is a rate limit of fifteen requests per fifteen minutes, so clients should additionally make sure to handle a rate limit response (HTTP status code 429) by retrying in fifteen minutes, otherwise a similar problem will arise for the rare user who has more than 75,000 blocks. If you develop a client, email firstname.lastname@example.org and I can provide temporary access to a testing account with >75,000 blocks.