@torn2 @Jenkler The only related functionality that I added would be logic that automatically generates a new session on login. It means that every time you log in, your cookie is updated, and any other browser tabs would no longer be valid.
There is logic on the client-side to handle that, so if you do use multiple tabs, you will just see a modal pop up telling you to refresh the page (or hitting "OK" will do that also).
By any chance are either of you using multiple tabs to browse your forum?
Not really, I mainly only use one tab for my forum but will have other tabs open to different sites that I may switch between. I'm going to leave two tabs open for a while and come back to one to see if it reproduces. I honestly do not get it this way very often. The other way is when posting images and have the imgur plugin. And I never actually log out.