This may have been mentioned before, but I get the full signal coming through my cab and desk (I run powered to cab and main to FOH at the same time) when powering down.
Might not seem much, but it can pretty embarrassing when, after finishing a show I'll put it into tuner to mute it, only to have my guitar squealing at me (and through the PA if the channel isn't muted) as soon as I turn it off.
There re must be a way to mute the signal altogether on power down? So far it's just made me look a bit of a tit, but I'm sure this could potentially damage something if it's unexpected, especially through the PA.