Posted the REE issue to the REE google group as well: https://groups.google.com/forum/#!topic/emm-ruby/EBkMfFpk40A The regex used on line 61 in rack/backports/uri/common.rbcauses REE to hang for certain strings. I only know because it happened in production over and over the past week and today, with help, I was finally able to track it down. Not sure if this is really an issue with Rack or just with REE, but it will affect anyone using Rack 1.3.0 and REE so I thought I would mention it here as well.