Does the version of a SQL Server Witness matter? 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.