Different thumbnail

I try posting an image, then when I see my post in the thread the thumbnail is something completely different, often an image I've never seen before.
I delete the post right away, so I don't know if the full image itself is different also, but I'll check next time it happens.
This has happened to me more than once today. What gives?

Posting at the very same time (down to the nano second!) is rare, but it did happen to me once! This happening to you multiply times seems unlikely, are you posting on /b/?