While trying to save a file out of Nano the other day, I got an error message saying "XOFF ignored, mumble mumble". I have no idea what that's supposed to mean. Any insights?
You pressed Ctrl+S instead of Ctrl+O to save the file. Ctrl+S is an old flow-control key combination to pause the transmission, and stop scrolling, of data to a terminal (internally, the code sent is called XOFF). Ctrl+Q (XON) is the complement to start transmission and resume scrolling.
Nano ignores these code since it doesn't use scrolling. The muttering is likely a result of its frustration that you didn't remember to use Ctrl+O to save.
You typed the XOFF character Ctrl-S. In a traditional terminal environment, XOFF would cause the terminal to pause it's output until you typed the XON character.
Nano ignores this because Nano is a full-screen editor, and pausing it's output is pretty much a nonsensical concept.
As to why the wording is what it is, you'd have to ask the original devs.