If a field is mostly accessed in a synchronized context, but occasionally accessed in a non-synchronized way, the non-synchronized accesses may lead to race conditions.

Ensure that the non-synchronized field accesses are made synchronized, if required.

In the following example, counter is accessed in a synchronized way in all but one cases. If modifyCounter is called by a large number of threads that are running concurrently, the value of counter at the end of each call may not be zero. This is because the non-synchronized statement could be interleaved with updates to the counter that are performed by the other threads.

To correct this, the last statement of modifyCounter should be enclosed in a synchronized statement.

Java Language Specification: Synchronization.