Rubocop is a "linter" for Ruby that checks for style problems and the like. I've heard people complain that it's finicky.
Tonight this happened:
Rubocop: 1 error (autocorrectable).
Me: do it.
Rubocop: 1 error.
Yeah, Rubocop corrected one problem and, in doing so, introduced another. Fun times! (Its change added a few characters to a line, which was apparently just enough to push it over a line-length check.) And that wasn't autocorrectable.
It's always something...