My plugin collaborates with an application through LrSocket. Unfortunately, when LR shuts down, it only calls LrShutDownApp after the socket no longer functions. I've tested this by sending a message to the application by LrShutdownApp and LrShutdownPlugin. The message gets sent when LrShutdownPlugin is called, but not when LrShutdownApp is. This creates significant problems for any plugin that has an associated application, as you can't get an orderly shutdown of both.
Fix--call LrShutDownApp procedures earlier in the shutdown process, prior to killing the sockets.