Well I wouldn't say that the function had been categorically identifed - but even then only in the context of features or plugins that are not enabled or installed on my system.

If I hold Play on the SB, the player hangs. Only a reboot will free it - and even then sometime I have to restart SC (7.2.1) to get the player back.

I can see nothing "mix" related in the web UI, nor on my Duet.

In the SC logs I can see lots of these entries:
[08-11-13 20:51:27.1099] Slim::Control::Request::execute (1888) Error: While trying to run function coderef [Slim::Control::Commands::buttonCommand]: [Can't use string ("-1") as an ARRAY ref while "strict refs" in use at /<C:\Program Files\SqueezeCenter\server\squeezecenter.exe>Slim/Player/Client.pm line 1016.
]
[08-11-13 20:53:46.5076] Slim::Control::Request::execute (1888) Error: While trying to run function coderef [Slim::Control::Commands:owerCommand]: [Can't use string ("-1") as an ARRAY ref while "strict refs" in use at /<C:\Program Files\SqueezeCenter\server\squeezecenter.exe>Slim/Player/Client.pm line 1016.
]
[08-11-13 20:53:47.4765] Slim::Utils::Timers::checkTimers (210) Error: Timer failed: Can't use string ("-1") as an ARRAY ref while "strict refs" in use at /<C:\Program Files\SqueezeCenter\server\squeezecenter.exe>Slim/Player/Client.pm line 1016.
[08-11-13 20:54:05.0181] Slim::Networking::IO::Select::select (251) Error: Select task failed: Can't use string ("-1") as an ARRAY ref while "strict refs" in use at /<C:\Program Files\SqueezeCenter\server\squeezecenter.exe>Slim/Player/Client.pm line 1016.
[08-11-13 20:54:05.0190] Slim::Networking::Slimproto::client_readable (425) client_readable: Client not found for slimproto msg op: STAT
[08-11-13 20:54:05.0194] Slim::Networking::Slimproto::client_readable (425) client_readable: Client not found for slimproto msg op: STAT
]