This was originally part of MDL-39769. The issue has been fixed in Clean (2.6) but I'm not quite sure when. It does not affect desktop browsers even if you resize the window to phone-like sizes.

If you use the phone to take a photo, for example while posting on a forum, the image overflows the tinyMCE editor. The editor still works and the forum post saves fine. It just looks weird while you're editing.