Since upgrading to Xcode 4.2 I’ve regularly encountered an error where attempting to debug an app fails with the message “Unknown packet reply: “timeout” to environment packet.” Restarting the device does not consistently solve the problem (though it sometimes does), nor does restarting Xcode or rebooting the development machine. Cleaning and recompiling also does not fix the problem.
It’s not limited to the project; once it happens I can no longer launch any apps through Xcode.
The device is an iPad running iOS 4.3.5; the development machine is a Macbook running Lion.
I still have no idea what was causing this before, but I haven’t seen it since upgrading to Mountain Lion and Xcode 4.4.1 so I’m answering the question to mark it closed.