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THE PROBLEM
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While developing a web app with Ruby 1.8.7-p299, Rails 2.3.5 and Paperclip, IOErrors began occuring intermittently
after files were uploaded and/or destroyed.

The problem was first noted only in development and test.  In the production environment, they never occurred.
Eventually, IOErrors began appearing in production as well.

We found the problem very difficult to diagnose because the exception stacktrace was never consistent.  The IOError
would be raised by pieces of code completely unrelated to file uploads, and/or by rescue statements.

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THE CAUSE
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Some research revealed that many people have experienced this problem (using other frameworks as well,
not just Rails).  The root cause appears to be a race condition between the Ruby garbage collector
and the Tempfile class finalizer.

This explains the intermittent nature and odd stacktraces.  The IOError only occurs when the ruby garbage
collector runs and a Tempfile object has already been closed.

The scripts in the cause/ folder reproduce the error in several different scenarios.  Using ree-1.8.7
prevents the IOError in the most common case, but not all.

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THE FIX
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The error is reliably reproducible using both ruby-1.8.7-p299, ruby-1.8.7-p302, and ree-1.8.7 and likely
affects other versions as well.

The original line of code is:
    tempfile.rb:167: tmpfile.close if tmpfile 

Intermittent IOErrors can be prevented by changing this line to
    tempfile.rb:167: tmpfile.close if tmpfile && !tmpfile.closed?

Adjust the exact line number as necessary if using a different version of Ruby.

The tempfile_monkeypatch.rb file is a Rails initializer we added to our application which has successfully
corrected the problem.  No side-effects have been noted (yet).