When forking a child process, while simultaneously there is a thread which is blocking on an IO operation, if that IO instance is closed in the child process, it will try and interrupt the blocking operation which no longer exists (in the child process).

r, w = IO.pipe

thread = Thread.new do
  r.read
end

Thread.pass until thread.status == "sleep"

fork do
  r.close
end

w.close

Process.wait
thread.join

For a little bit of background: when forking, all native threads except for the one doing the fork operation go away. That means stack allocated data structures on those threads become invalid, e.g. struct rb_io_blocking_operation. We track the fork generation to know whether the entries in the blocking_operations list are valid or not, and if the fork generation mismatches, we clear the list. This is the same technique used by Queue.