Very rare error message. One time I generated a "That's not your stream to edit" message when I made a Settings button and was trying to edit stream settings on a stream I didn't own.
Users can get these types of "That's not your x" error messages by two ways that I can think of.
1) editing the page html to create and click an invalid button or link to something the account doesn't own.
2) switching between user accounts without refreshing the browser page, and then using an invalid button or link to something the logged in account doesn't own.
