Are you using GAE? You might be hitting this: http://code.google.com/p/googleappengine/issues/detail?id=8201 http://apache-tapestry-mailing-list-archives.1045711.n5.nabble.com/T5-and-AppEngine-1-7-3-dev-server-IllegalStateException-td5717683.html
Fix here: https://bitbucket.org/akochnev/tap5-gae-utils/wiki/Home If not, it might be caused by the GZIP filter. Can you try setting "tapestry.gzip-compression-enabled" to false? http://tapestry.apache.org/configuration.html#Configuration-tapestry.gzipcompressionenabled
