While hiking up to Rabun Bald yesterday, we saw lots of butterflies and the phrase flutter-by kept popping up in my head as being an unusual turn of a phrase where it actually describes the thing in question quite well.
Is there a term for this? Can you think of any others that do the same thing?
There was the SR-71 Blackbird, which was called the RS-71 until the President misspoke. 
As to the technical term, I’m ignorant.
It’s a spoonerism, which is a form of metathesis. I don’t know if we differentiate between spoonerisms that actually mean something (flutterby being a strong example) and ones that create nonsense words.