Last modified: 2013-10-12 00:04:58 UTC
An empty string, a space, zero, etc. Edits like http://commons.wikimedia.org/w/index.php?title=File:%22%D0%A3%D1%80%D0%BE%D1%87%D0%B8%D1%89%D0%B5_%D0%93%D1%83%D1%80%D0%B1%D0%B8%22,_%D0%BA%D1%80%D0%B8%D1%97%D0%B2%D0%BA%D0%B0_%D0%A3%D0%9F%D0%90.jpg&diff=prev&oldid=79691669 should not happen.
The national tag templates should implement some filter mask which should detect and eliminate strings which are not in expected form (only numbers, number of digits, a position and a number of letters, slashes, hyphens or spaces, a range of numbers - ignore zero and empty values etc.).