I just fixed a rather insiduous 2.3-only problem in fcgi: Apparently, the 2.3 socket module does not define the SHUT_* constants for the shutdown() syscall. This caused an attribute error that was silently being ignored. The end result was that the FastCGI socket connection with the server was not being closed - this caused output to hang in certain situations.
I didn't see the problem because as I said previously, I run 2.4 everywhere - even on my Macs (which normally come with 2.3 out-of-the-box).
Anyhow, big thanks to Eugene Lazutkin for bringing the problem to my attention, helping to isolate the problem (since it was unreproducible on my end, I had a hard time believing it was fcgi's fault

), and finally, providing access to an environment where it was reproducible 100% of the time (which allowed me to figure it out).