From the "I'm glad I didn't discover that the hard way" department...
One of our tipsters alerted us to the fact that there was an Apple shortcut which, used in the Finder, opens a network share. But, used in the Mail application, the same shortcut deletes mail.
I thought I'd check it out. And indeed Apple-K used in the Finder opens the "Connect to Server" window. I opened up Mail, tried the same Apple-K command again... and got a warning message "Are you sure you want to permanently erase the deleted messages in 'POP Account'? This cannot be undone..."
So yes, in Mail the Apple-K command permanently compacts/empties deleted messages. Our tipster complained that the double-up of commands is particularly worrying since the Finder Apple-K also pops up a dialog that you often click straight through. At least it's only deleted mail you'd lose if you ignored the alert.
So have you come across any "shortscuts that weren't" recently? Post 'em in comments please!
Oh, and thanks Anth and Toby for the tip!