A mix up of the phrase “six of one, half dozen of the other”.
Found in this tweet by @sarahkcomedy.
My girlfriend says every known phrase incorrectly, but with absolute unwavering confidence. Some of my favourites:
"It's six of one, a dozen of the other" "It swings in roundabouts" "I'm stressed to the nines" "Lets get this boat on the road" "Like a bulldozer in a china shop"