That looks like what happened looking at the difference here:
17/05/2010 - slash
17⁄05⁄2010 - solidus
The preview I see while making this post looks similar to his screenshot, where Windows isn't giving the solidus much room to the point where it's almost touching the numbers.
(For Mac users, it's Option-Shift-1.)
If it is a solidus, thats a pretty clever trick. The OS X Finder does something different to enable forward slashes (for compatibility with the classic Mac OS). In the GUI, a / can appear in a file, but under the hood on the Unix side, it's translated to a : . Makes me wonder how it does the translation though if it were to be copied into OS 9, as : is the folder separator in that OS. OS X Finder still bans : from being manually entered in a file name.
*edit, hrm, the BBS blew it up when even I posted. Lets try the admin cheat of HTML post mode*