incorrect proof of work

[5chdn]

If you have the latest client you dont have to worry.

This log message only says there are blocks out there which can not be validated (from the old abandoned chain).

[jetsfan11]

I have the newest client. I'm seeing an error in my log that says...
ERROR: Acceptblock() : incorrect proof of work.

Is this normal or do I have a problem? Does it matter if generate is set true or false? Client says it is mining either way.
Thanks in advance. I just want to make sure I'm not just wasting CPU cycles.