Users are logged out when viewing nodes

Question

I'm having a really strange problem with my test website. When users (both admin and non-admin) visit node pages, they are logged out and are being logged in again as the user who commented last on the node.

I've no idea what causes this behaviour, as I've never experienced this before. I don't have any caching modules and clearing caches multiple times brings no solution. What can cause this problem?

Answer 1

I finally found the reason for this weird problem: my comments' template contained a misplaced global $user;. I don't know why this causes trouble, but at least it's fixed now. Hopefully, this can get people with a similar problem on the right track to find a solution.