insane fan speed readings from lm_sensors

[Lonni J Friedman]

Since its so quiet here (when is it not?), I'll drop a puzzle on you
fine folks to noodle on.

I recently started monitoring my system exhaust fan speed (RPMs),
mostly out of curiosity.  What I discovered has exposed a mystery that
I'm struggling to explain.  Every day between 04:00 & 10:00, the fan
speed readings start to rapidly oscillate between sane numbers (under
3000 RPM) and insane numbers (over 10,000 RPM).  For the rest of the
day, the readings are perfectly normal & reasonable.  Its always
during that same time window.  There's no way this exhaust fan is even
capable of spinning at the rate that is being reported.  This has to
be an erroneous reading.

Here's a graph of what I'm talking about:
https://imgur.com/erXGz4J

There's nothing in dmesg or /var/log/messages to explain this
behavior.  I've looked at every cronjob running on the system for some
bizarre correlation, and can't find anything.

I'd like to think that if this was a flaky sensor, it wouldn't be so
reliable in when it happens.  So something else must explain the
weirdness.  Anyone have ideas/suggestions?