If a biblio has several copies which are numbered 1,2,3,4 etc, when sorting the holdings table by copy number it results in the display of 1,10,11,2,20,21 etc. This is on the cgi-bin/koha/catalogue/detail.pl page.
I have seen this one, and it's a tricky one, because I don't think copy number is strictly a number. It's technically text and could be something like "c. 1" or "c. 2", so lexicographic sorting gets used by default.