My Mother (when I'm looking for something in plain sight):
"If it was a bear, it would've bit you!"
Finally one day I said "If it was a bear, it'd be a lot bigger!"

and:
"If you go outside with wet hair, you'll get polio like Franklin Roosevelt!"
until one day I said "Yeah. And he never amounted to anything after that, did he?"

also:
"If you fall out of that tree and break your leg, don't come running to me!"