The analog wall clocks in high skool were all wired to central time
controller. Curious as to how it worked, I dragged an oscilloscope
into the main hallway to clip onto the only accessible wires I could
find. Every 15 minutes, a sync pulse would appear on the line,
resetting the clocks to the nearest 15 minutes. Every hour, two
pulses would reset the clocks to the nearest hour. At noon and
midnight, 5 pulses would reset the clocks to midnight. Unfortunately,
gathering this intelligence required almost constant monitoring, which
attract too much attention. I was caught before I could make the
clocks run backwards. Not a great start for my first attempt at
hacking.

There's a classic "Carl and Jerry" story about this. The scholl clocks
aren't keeping good time, and -- for no obvious reason -- they run faster
when it's raining.

It turns out that the janitor's vacuum cleaner put out a lot of line noise,
and even more when it was sucking up water.