I hope this is the right place for this issue (and not in the Kramdown issues list)
Reproducing it is simple:
install the current kramdown gem with jruby and run
> require 'kramdown'
> puts Kramdown::Document.new('aéb').to_html
It will run in an endless loop whenever there are special chars like é or ü or ß in the string.
With Ruby 1.9.3 it works fine.
Currently I'm not able to get more information by looking at the JVM's stacktrace.
Maybe this relates to https://github.com/rtomayko/rdiscount/pull/58 where Charly said: "and rdiscount_test.rb appears to hang (possibly a JRuby bug)."