When I click the middle mouse button on a link in the preview area of the editor, in Chrome, this opens the link in a new tab and pastes the clipboard content at the cursor position. The expected behavior would be to open the link in a new tab, only: a middle-click in the preview area should never paste. A middle click not on a link does not paste.
Observed in Chrome 27.0.1453.93 and 28.0.1500.95 under Linux (Debian wheezy, Ubuntu 12.04) and in some prior versions, I've forgotten when this started but it isn't new. Reproduced in incognito mode, not logged in and with no extensions. In my tests, this happens systematically when the editor is in a separate tab (whether I'm editing my own post or suggesting an edit), but not when the editor is inline. Not reproduced in Firefox.