> I have Apache configuration like this:

>     <Proxy balancer://production>
>     BalancerMember http://127.0.0.1:6543
>     BalancerMember http://127.0.0.1:6544
>     </Proxy>

>     ProxyPass / balancer://production/
>     ProxyPassReverse / balancer://production/

> Ports 6543 and 6544 are pserve processes using Waitress.

> From time to time, requests to pages will fail with 502 error (roughly 1
> out of 3000 requests). When that happens, there will be logs like this from
> Waitress:

> 2012-08-07 18:06:28,607 ERROR [waitress][Dummy-3] Exception when serving
> /cases/new
> Traceback (most recent call last):
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\channel.py",
> line 329, in service
>     task.service()
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 173, in service
>     self.execute()
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 412, in execute
>     self.write(chunk)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 285, in write
>     channel.write_soon(rh)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\channel.py",
> line 311, in write_soon
>     self.outbufs[-1].append(data)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\buffers.py",
> line 246, in append
>     buf.append(s)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\buffers.py",
> line 54, in append
>     read_pos = file.tell()
> ValueError: I/O operation on closed file.
> 2012-08-07 18:06:28,608 ERROR [waitress][Dummy-3] Exception when servicing
> <waitress.channel.HTTPChannel 127.0.0.1:61326 at 0x7883198>
> Traceback (most recent call last):
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 76, in handler_thread
>     task.service()
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\channel.py",
> line 342, in service
>     task.service() # must not fail
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 173, in service
>     self.execute()
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 320, in execute
>     self.write(tobytes(body))
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\task.py",
> line 285, in write
>     channel.write_soon(rh)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\channel.py",
> line 311, in write_soon
>     self.outbufs[-1].append(data)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\buffers.py",
> line 246, in append
>     buf.append(s)
>   File
> "c:\virtualenv\production\lib\site-packages\waitress-0.8.1-py2.7.egg\waitre ss\buffers.py",
> line 54, in append
>     read_pos = file.tell()
> ValueError: I/O operation on closed file.

> We didn't encounter this error with Apache fronting CherryPy. It only
> started happening after switching to Waitress.