If you call require in one thread, and while the code is being executed, call require in another thread, the second thread will hang.

This is changed behaviour from 1.7.4.

Test case: https://gist.github.com/bensummers/6484475

When running the test case:

$ jruby -v test.rb
jruby 1.7.4 (1.9.3p392) 2013-05-16 2390d3b on Java HotSpot(TM) Server VM 1.7.0_25-b15 [SunOS-x86]
Boo!
(exits normally)

$ jruby -v test.rb
jruby 1.7.5.dev (1.9.3p392) 2013-09-07 06d6243 on Java HotSpot(TM) Server VM 1.7.0_25-b15 [SunOS-x86]

(hangs, never returns)