A phrase with the same meaning as the "shit hits the fan" but from more original origins. A midden was a pile of general rubbish, generally containing excreations. A windmill is self-explanatory and so when the midden hits the windmill you are all going to get showered in shit.
Man 1) The boss found out that the virus on his PC was our fault
Man 2)The job's not worth it. Lets get out of here before the midden hits the windmill