A binary synchronization transition of a low priority process  is 
enabled even if a binary synchronization transition of a higher priority 
process using the same channel is possible.

Problem description:
This problem occurs only when also a delaying transition in a third 
process is enabled.
It is visible in the last transition in the attached  example trace.

Example model: see attachment 1 [details]
Example trace: see attachment 2 [details]

Created an attachment (id=214) [details]
Example model

Created an attachment (id=215) [details]
Example trace

Fixed in rev. 4378 (4.0.9) and 4379 (trunk).
We have changed the semantics because it was inconsistent.
Before, priority was resolved by comparing the priorities of
the processes involved in two transitions by descending order.
Now, it is resolved by considering the highest priority process,
which is in line with priority ceiling protocols.

In this example, it is normal to have the synchronization with
the new semantics (though not normal with the old one).