In article <4a9382cec12f4_212ed99b56c6225 / redmine.ruby-lang.org>,
  Daniel Azuma <redmine / ruby-lang.org> writes:

> IO#select (Kernel#select) fails when run on different sets of IO objects in different threads. This affects release versions 1.8.7p160, 1.8.7p173, and 1.8.7p174. It does NOT seem to affect recent versions of 1.9.1 that I have tested. It also does NOT affect release version 1.8.7p72. I have not tested 1.8.6 versions. The repro steps have been tested mostly on Mac OS X 10.5.8 on an Intel-based MacBook Pro. I have, however, seen similar behavior on a recent Fedora Linux i686.

The problem is fixed in the latest 1.8 branch.  
-- 
Tanaka Akira