It looks like the performance improvements mentioned on the JRuby blog probably broke exception backtrace generation in some cases. This affected RSpec, causing the following bug: rspec/rspec-core#2299. I can reproduce the problem by running this very simple script in JRuby:

begin
  raise
rescue ArgumentError
rescue StandardError => e
  if e.backtrace.nil?
    puts "Rescued an exception with no backtrace!"
  else
    puts "Looks good"
  end
end

Here is a shell session showing my environment and the output of that script:

$ jruby -v
jruby 9.1.7.0 (2.3.1) 2017-01-11 68056ae Java HotSpot(TM) 64-Bit Server VM 25.121-b13 on 1.8.0_121-b13 +jit [mswin32-x86_64]
$ uname -a
MINGW64_NT-10.0 URANIUM 2.6.0(0.304/5/3) 2016-09-07 20:45 x86_64 Msys
$ jruby simple.rb
Rescued an exception with no backtrace!

MRI behaves as expected and prints "Looks good" when it runs that same script.

For anyone affected by this, a workaround is to add => _ to the first rescue statement.