I’d tag this beta10, but that tag doesn’t exist yet. Thankyou @downey
Anyway, if you have two tabs open, and both tabs have their composers open, pasting an image from the clipboard in one tab pastes the image in both tabs’ composers.
Tested on Windows 8.1, Chrome 43; not sure if this is a Chrome thing or a Discourse thing yet.
Update: Also appears to happen on Firefox 38 on Windows 7