This is likely coming from deeper in JRuby, like from Regexp logic, but here's a quick script based on the Windows failure from ruby/strscan#25:

require 'strscan'
s = StringScanner.new("foo bar baz")
s.scan(/(?<a>\w+) (?<b>\w+) (\w+)/)
s["\u{30c6 30b9 30c8}"] rescue p $!

On non-Windows platforms, this last line prints #<IndexError: undefined name <テスト> reference> but on Windows the Japanese characters are mangled into undefined name <\u00E3\u0192\u2020\u00E3\u201A\u00B9\u00E3\u0192\u02C6> reference

This is the sole remaining failure for the JRuby strscan support. Note that this is only a different in the error message on Windows; the error itself is firing correctly on all platforms.