Optimistic lock failed : object has version .. instead of ..

I do basic storage and retrieval of Record class.

When I stress the retrieval/storage with Apache JMeter sometimes (at some requests) I get this exception.
 

[ObjectDB 2.5.0] javax.persistence.OptimisticLockException
Optimistic lock failed for object entities.Record#'1405/838/2011/121' (object has version 125 instead of 126) (error 673)
at com.objectdb.jpa.EMImpl.beforeCompletion(EMImpl.java:381)

...

Thank you!

Optimistic locking is explained in this manual page.