When the chat ads in the sidebar display the last activity information (e.g. "n mins ago - username") the code makes the assumption that there's always a message object to get that activity from.

This apparently isn't always the case though, so the call `chat_ad.userLink(e.messages[0])` results in an error message about the passed parameter being undefined when the code tries to access the `.user` property on it. This error can cause other things from running properly (like my userscripts), though it does appear to happen late enough in the page not to impact any important site functionality.

Alternatively, it's possible that the bug lies in the fact that there weren't any message objects, and the code works off the assumption that there always should be. This was the relevant JSON for the room:

    "name":"Your room",
    "id":355,
    "lastPost":1297627212,
    "posts":1,
    "users":[ /* Nyuszika7H's user information */ ],
    "messages":[],
    "events":[]

The last post timestamp listed seems to coincide with [this message](http://chat.meta.stackoverflow.com/transcript/message/510990#510990), even though it doesn't appear in the messages array (despite there being a post count of 1).