Comment #4 on issue 260 by dorma...@rydia.net: memcached big bug http://code.google.com/p/memcached/issues/detail?id=260
To get a backtrace you can either take a core dump from memcached, or start it under gdb.
"gdb memcached etc" in a screen session. then when it crashes, run "bt" and "thread apply all bt" and give us the output of both.