Description:
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When php encounters a FATAL error and output buffering is ON, the child process that handled the script no longer outputs anything, even on subsequent requests to working scripts. If the below script is run, it returns a completely blank page. If a non-erred script is then run, the non-erred script also returns a blank page. It's only fixed by killing the offending child process. With output buffering disabled, everything works as expected and the child process doesn't need to be killed.

Test script:
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<?php
echo "Working happily";
call_nonexistent_function();
?>

Expected result:
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Working happily
FATAL ERROR: Call to undefined function call_nonexistent_function() in.....

Actual result:
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Blank page - nothing at all.