Comment #7 on issue 127 by [email protected]: incr/decr operations are not thread safe.
http://code.google.com/p/memcached/issues/detail?id=127
This has finally been fixed and merged into 1.4.6-rc1, please reopen if it's broken.
1.6 has it fixed via another approach, but I'm not sure if that work is completed as of now.
