Since the release of version 9 we get a "You don't have permission to transition this issue" message, as illustrated in this kb article.

The KB article "Possible causes to error you dont have permission to transition this issue ticket" indicated there are at least four possible causes:

• User without permission to transition
• Status without outgoing transition
• Workflow status/transition with property making it impossible/restricting any group
• Workflow conditions allow only a specific group to perform transitions

All of the above items' investigation starts by looking at the workflow transitions possible from the current issue's workflow status and available transitions. 

It will be very helpful for the users if we display the possible transitions even though they are unavailable due to a permission problem.  Here is a sample mock-up:

The information can help users start the investigation by looking into particular transitions and presenting more details on the error.