A Famous Idiom!
You should never cry over spilt milk.
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Unless, of course, you are stranded on a deserted island and a magical genie shows up and offers you a lifetime of pleasure, fame, and fortune in exchange for your last glass of milk and you leap with glee and nearly spill your milk, then after you sigh in relief that it hasn't spilled, you step on a clam shell and drop the glass, causing the genie to disappear and leaving you to live out your days on a diet of coconuts and dead seagulls.
:D