Can you have a SQL Server witness in a mirror setup which has a higher SQL Server version than the mirror?
Ie. the mirror is on SQL Server 2012 Standard and the witness uses SQL Server 2014 Express.
From BOL :
•For a mirroring session to be established, the partners and the witness, if any, must be running on the same version of SQL Server.)
Note: Express edition of the same sql server version is supported.
Edit: based on the comment from @Raidri
Technically it is possible to have a witness with a higher version of sql server than the 2 partners, but it is highly recommend and supported that all servers have the same VERSION of sql server running - edition can be different.
Except during an upgrade from an earlier version of SQL Server, the server instances in a mirroring session must all be running the same version of SQL Server. For example, a SQL Server 2008 witness is supported when you are upgrading from a SQL Server 2008 mirroring configuration but cannot be added to an existing or new SQL Server 2008 R2 or later mirroring configuration.
The two partners, that is the principal server and mirror server, must be running the same edition of SQL Server. The witness, if any, can run on any edition of SQL Server that supports database mirroring.
You can, and upgrade scenarios are the ones for which it makes sense for the scenario to be supported.
However, there would probably be no testing at all in Microsoft for this scenario, except during an upgrade. So if you expect SQL 2008 R2 witness working with SQL 2014 partners, you are risking yourself into an untested scenario. Protocol changes are small, but not null.