pg_dump failed with error code 255, but I don't see why

v9.6.2.23

I always run pg_dump with the --verbose option, and the bash script echos 
$?, so there's a 25,664 line log file for me to examine which explicitly 
shows the return code.

But however much I search the log file for strings like ERROR, FATAL, WARN, 
INFO, and "archiver", I don't see any problems.

What else should I look for?

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