Since upgrading my RedHat 5.1 wu-ftpd to version 2.5.0 I occasionally
received through syslog a message like this:

Apr 19 12:52:25 alpha ftpd[5104]: exiting on signal 11

subsequent upgrades to wu-ftpd 2.6.0 did not resolve my problems.
I always used RedHat precompiled versions (which also "featured" the long
known ftpwho error message on RH5.x, already featured in Bugzilla but never
solved).
Now I have downloaded and rebuilt 2.6.1-1.src.rpm and the message is:

Jul 21 19:59:14 alpha ftpd[7296]: exiting on signal 11: Segmentation fault

the ftpwho error is gone now (maybe autoconf'd build process figured out
the right ps syntax) but the sigsegv seems more frequent too. I'm running a
low traffic web/ftp site and the access rate is actually slowing down much
now (summer time): so the sigsegv rate is raising!
I'm running a pure RH 5.1 system with all the upgrades (now released
officially only for 5.2...) and kernel 2.0.38 with Solar Designer security
patches.
Wu-ftpd runs from inetd with the line:

ftp     stream  tcp     nowait  root    /usr/sbin/tcpd  in.ftpd -l -a

and a somewhat tweaked ftpd configuration (but no virtual hosts).
Judging from the access logs, real and guest users seems to have been
unaffected by the problem and the anonymous ones who get caught by sigsegv
are in vast majority using IE4 (but some GetRight report too).  There seem
to be no other pattern.

Hope you can catch the bug (or help those kind souls finishing up the
latest proftpd release... ;> )