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On a non-Windows OS where _posixsubprocess is missing (subprocess uses the pure Python implementation), if the child fails with a Python exception and the exception message contains a surrogate character, message.encode() fails silently (exception while processing exceptions are just ignored).
Surrogates should be passed to the parent process: surrogatepass can be used for that. Attached patch implements this idea with an unit test.
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_posixsubprocess is not concerned because it writes an empty message for OSError (the parent process calls os.strerror() to get the message) or "Exception occurred in preexec_fn." (pure ASCII string) for RuntimeError.
On Windows, _subprocess.CreateProcess() calls PyErr_SetFromWindowsErr() on failure without the filename. So there is no surrogates here.