Water pilot The rate compass which was used to set the ships' course for the autopilot was installed just a little too close to the DC pump for the freshwater toilet, leading to a magnetic field interference issue that caused large swings in course when the toilet was flushed.
Bringing that back to the 737, in the NG and MAX, we can tell when the lav is flushed, airborne, because the cabin air pressure rate of change gauge shows about a 500 fpm drop, and then rise. (fortunately, the compass is a little farther away.)