HBO's "Luck" Memo Forget Dead Horses ...It's March Madness!!!
Hours after horse #3 keeled over ... the cast and crew of "Luck" received a sensitive memo urging them to remember one very important thing -- place your bets for the NCAA Basketball Tournament!!

TMZ obtained the "Luck" call sheet for Wednesday -- featuring shoot times for Dustin Hoffman, Dennis Farina, Nick Nolte and others -- and while it makes no mention of Tuesday's on-set tragedy ... it does state in bold type: "Don't forget to turn in your NCAA brackets ... by 7PM tonight"

That turned out to be the last call sheet -- "Luck" was cancelled late Wednesday ... and HBO's statement noted their "heartbreak" over the situation. Clearly, nothing soothes the soul faster than nailing the Final Four.

To be fair, March Madness does begin today -- and similar reminders were sent around millions of offices ... though we're guessing a horse didn't drop dead in most of them.