Sorry I can't find a smaller code to reproduce this, but this happens only in jruby 9.0.5.0,
and 9.0.4.0 works fine. Here's the code:
https://github.com/godfat/rest-more/blob/rest-more-3.4.0/lib/rest-core/client/facebook.rb#L8
Builder.client would return a new class, and the name should be set to RestCore::Facebook.
Surprisingly, if you change the constant to FAcebook, it works, and so on so forth.
Interestingly, Github won't work either.

I suspect this was due to that the constant might be considered set somewhere else.

This bug caused this error:
https://travis-ci.org/godfat/rest-more/jobs/106942941#L310
Because we can't serialize anonymous class (which shouldn't be).