Looks to be a corner case not covered by existing tests prior to now.

“This release contains a bug fix about Refinements and Module#prepend. The mixture use of Module#refine and Module#prepend to the same Class could cause unexpected NoMethodError. This is a regression on Ruby 2.3.2 released last week. See [Bug #12920] for details.”

Presumably it was discovered in the wild, fixed, test cases written, and that fix was pushed. Time taken: 6 days. https://www.ruby-lang.org/en/news/2016/11/15/ruby-2-3-2-rele...

[Bug #12920] https://bugs.ruby-lang.org/issues/12920