what more than likely happened is you deleted a member profile, but not out of the index.
there's the file for each member, but also a list that identifies the name with that number, in memberlist.cgi
check the file and make sure that the same number of members there are the same as the number files.
in other words, if thelast member that registered was 0000023, make sure that in the index, the last number is 0000023, and not 0000024.