From a quick glimpse on twitter, people couldn't make changes to any of the accounts they were seeing. This points in the direction of this being a caching bug

From a quick glimpse on twitter, people couldn't make changes to any of the accounts they were seeing. This points in the direction of this being a caching bug; you request your homepage, and get the homepage of whichever user was placed in the cache last. Most of the time in these situations it's not an application-code issue (per-se), as much as a "shared global state" issue.