If you search/replace the german ligature sz/ß, you will also find "ss" what is very anoying, for searching html-files with cla"ss"-tags.
Version: 0.20 (Ubuntu 11.10 repositories)
I assume your locale is set to German. If so the Unicode match rules say ß matches ss as well. You might be able to use regular expressions to prevent this match.
Not a bug, as annoying as it may be, Unicode defines that ß match ss, see ftp://ftp.unicode.org/Public/UNIDATA/CaseFolding.txt mapping for 00DF, note also the example in the initial comments.