I often see this error coming by in the log files. Any ideas on what and how. My app seems to work fine but sometimes this error pops up in the log files. It seems to happen only with bots and not normal browsers but I’ll have to double check this to be sure.

I have the same problem!

100 errors per day!!!

I was having the same problem, and it turned out to be an issue with mbstring breaking the Encrypt library.  I found a solution (and talked about my problem) in this post:

http://ellislab.com/forums/viewthread/91456/#758252

I have the same problem too

but I think this caused by the php configuration
I will try to set more memory for the php and check this and if success I will be back to tell you

The problem may also appear when you’re using UTF-8 and the ci_sessions table has incorrect charset (ex. latin1) set.

It can also happen if you are only using cookies for storing session data and the (serialized) data exceeds the 4k max that cookies can store.  It breaks the serialized array being stored as the string gets truncated, so it can’t unserialize it.