I noticed this once when accidentally sharing connections around. Could that be the case ?

What sort of commands are you running ? Could you be seeing this problem ?
http://www.mail-archive.com/user@cassandra.apache.org/msg04831.html

Aaron


On 29 Jul, 2010,at 12:47 PM, Jianing Hu <jianing@gmail.com> wrote:

We recently migrated part of our MySQL database to a 3-node Cassandra
cluster with a replication factor of 3. Couple of days ago we noticed
that Cassandra sometimes returns the wrong data. Not corrupted data,
but data for a different key than the one being asked for. This error
appears to be random and intermittent, and happens for maybe every 10K
reads. I'm working on a test suite that can reproduce this. Meanwhile
just wanted to ask if anyone has seen this problem before? I've tried
consistency levels of both QUORUM and ONE, and see the issue with
both.

Thanks,
- Jianing