Music file-sharing service Napster implemented a filtering system on Sunday night, meant to block users from downloading banned song titles, but by Monday afternoon users found they were able to bypass the system in some cases.

Unfortunately for the firm, as it had warned, one of the flaws of the system is that it can filter out only exact file names - songs with file names that have been misspelled or made up by users won't be blocked.

Given Napster's search engine can track down song titles even where a name is slightly misspelled or altered, this left users able to access some songs that were supposed to have been blocked. The Napster representative provided the names of three songs that are on the filtering list. Two of those three songs could still be accessed by users Monday afternoon.

Metallica's Enter Sandman, for example, was easily obtainable without needing to modify the search query. Over 100 users were offering the song on just one of Napster's servers under titles including 'Inner Sandman', 'Enter the Sandman' and simply 'Sandman'.

Another song on the filter list, Billy Joel's Piano Man, was available on the same server listed under 'Joel, Billy' followed by the title 'Piano Man' or 'PianoMan'. The song had apparently not been blocked because Billy Joel's name appears in reverse in the file name.

"The filtering engine has to be limited by definition, or else it would block out more songs than intended," said Malcolm Maclachlan, an electronic media analyst with IDC. "I think everybody knew that there would be a cat and mouse game between the users and the filtering service."

IDC is part of the IDG group, as is PC Advisor.