The test that's failing in mri/ruby/test_sprintf.rb:

  def test_hash
    options = {:capture=>/\d+/}
    assert_equal("with options {:capture=>/\\d+/}", sprintf("with options %p" % options))
  end

The failure:

ArgumentError: positional args mixed with named args
    org/jruby/RubyString.java:1221:in `%'
    /Users/headius/projects/jruby/test/mri/ruby/test_sprintf.rb:173:in `test_hash'

We do have code that raises this error around Sprintf.java:154 in Sprintf::Args#next, but I was not able to figure out the equivalent code in MRI (it doesn't seem to be there in the GETNEXTARG macro in sprintf.c nor in the logic for case 'p' in the main switch) nor does the test itself indicate where/when the change to sprintf was actually made.