Corrupt blocks in HDFS?

Few weeks ago we saw corrupt blocks in HDFS in jmx output (like http://namenode:50070/jmx). So we didn't start panic and run fsck. Hadoop fsck output said status HEALTHY and this was true.

...
    "PendingReplicationBlocks" : 0,
    "UnderReplicatedBlocks" : 0,
    "CorruptBlocks" : 5,
    "ScheduledReplicationBlocks" : 0,
    "PendingDeletionBlocks" : 0,
...

Result: Dont trust jmx everytime!

UPDATE: same situation is in HBase jmx and Regions In Transition (ritcount). We had some RIT, but jmx sayed "ritCount":0. JMX is bigger and biiger liars every day.