(Posted this as a reply in another thread but I wanted to ask specifically about the compass)
Last week I had a really scary crash, my quad was running in autopilot and suddenly decided to take it's own route. Within seconds and before I could react, it was completely off course and out of line of site.
As a last resort I activated the RTL failsafe and surprisingly the quad made it back but it was flying backwards, unfortunately it fell out of the sky before I could land it safely.
Analysis of the logs and on board video seem to indicate that there was a compass/heading error which slowly magnified as the mission progressed. While the logs and the telemetry data shown on the laptop indicated the heading was correct, the actual heading was slowly creeping away.
As such, while it flew perfectly fine on the first 3-4 waypoints it started to get it's bearings wrong soon after and this error snowballed into a crazy race across the sky ending in a crash.
I suspect there was something wrong with the compass, but after retrieving and repairing the quad, all bench tests seem to prove otherwise. This leads me to wonder if the compass can be compromised by external factors such as interference or magnetic fields around power lines etc.
Also, how essential is the compass for navigation? Somehow my quad made it back to launch even though it was pointing in the complete opposite direction, yet it still went off course halfway through the mission.