I've done the same thing; I think every brewer that kegs has it happen at least once. I accidentally closed my keezer lid on a picnic tap, the next day I found the keezer filled with about 4" of Apfelwine. A month's work completely down the drain (literally, unfortunately.) Now I double check, sometimes triple check - after I close the lid.
Primary 1: EdWort's Apfelwine
Primary 2: Yooper's Fizzy Yellow Beer
Primary 3: Surly Cynic
Bottled/Kegged: EdWort's Apfelwein, DC's BVIP, DFH 120 Minute Clone, Gratetful Dead Guy