When using the file naming script feature, Picard will first automatically replace illegal filesystem characters with underscores before the script has a chance to execute. The unintended result is that there are now unexpected underscores in the resulting filename. This can interfere with renaming scripts that would look for and replace the illegal characters.

Here's my example:

$if($gt(%totaldiscs%,1),%discnumber%.,)$num(%tracknumber%,2) - $rreplace(%title%,/,⁄)

The %title% of my audio file is:
Steve, the Emperor of Beer / from Super Barbecue III

Attempting to run the script outputs this:
06 - Steve, the Emperor of Beer _ from Super Barbecue III.mp3

The forward slash was automatically changed  to an underscore without any input or settings and this behavior is unable to be changed thus, my script will not catch or replace it. It should have been replaced with a fraction slash character, which is a suitable and possible replacement.

I would propose that the search and replacement of the illegal filesystem characters behavior occur AFTER the file naming script is executed, possibly with a warning dialogue showing what will happen to the resultant file.