[ pid=15278 thr=3078633296 file=ext/apache2/Hooks.cpp:789 time=2011-07-10 09:49:52.554 ]:
Either the vistor clicked on the 'Stop' button in the web browser,
or the visitor's connection has stalled and couldn't receive the data that Apache is sending to it.
As a result, you will probably see a 'Broken Pipe' error in this log file.
Please ignore it, this is normal.
You might also want to increase Apache's TimeOut configuration option
if you experience this problem often.
調べてみたところ、当方のWebサイトはRuby on Railsで構築していてPhusionPassenger(Ruby on Rails をApacheで動かす際に必要なもの)を使用しています。このPhusionPassengerが関係しているようです。
ただ、それ以上のことは調べてもよくわからない!