Me: "3 Gb downloaded. I'll let the last bit download and install it in the morning." [switches to login window]
Mac: "Ooh, look, he's switched to the login window. Guess he won't be needing this, then." [deletes download from ~/Library/Application Support/AppStore]
WTF? Why would Mac App Store do that? It's not like I logged out. (And even then, it should be possible to resume a partial download.) Grr...