Upload a file to mediawiki 1.18alpha (r79596)
Delete that file.
Reupload a file of the same name. A warning appears, which is helpful. In the upload form, at this point, the filename is still present under "Source filename:"
Click at bottom of form on "Ignore warning and save file anyway". Now the upload fails with message above the (now empty!) "Source filename:" : "The file you uploaded seems to be empty. This might be due to a typo in the file name. Please check whether you really want to upload this file."
Repeatable...
Version: 1.17.x
Severity: major