[root@x64test ~]# redis-cli sadd testsadd 123 (integer) 1 [root@x64test ~]# redis-cli sadd testsadd 234 (integer) 1 [root@x64test ~]# redis-cli SMEMBERS testsadd 1. 234 2. 123 [root@x64test ~]# redis-cli expire testsadd 10000 (integer) 1 [root@x64test ~]# redis-cli SMEMBERS testsadd
1. 234 2. 123 [root@x64test ~]# redis-cli sadd testsadd 456 (integer) 1 [root@x64test ~]# redis-cli SMEMBERS testsadd
1. 456 [root@x64test ~]#

This is expected behaviour even if it is a bit weird. Read the section "Restrictions with write operations against volatile keys" here http://code.google.com/p/redis/wiki/ExpireCommand to learn more.

As brampton indicated, this is expected behavior.