Years ago I chartered a boat with a buddy and we were using the autopilot... the AP worked fine until my buddy lingered at the companionway... suddenly the AP would swing us off about 15 or 20 degrees. Turned out the massive boat knife on his hip was the cause. The fluxgate was mounted near the companionway... and depending on exactly how he stood, the knife was enough to cause a swing in the AP.

Just an error though, not enough to do a 180 or anything.

We laughed thinking that if he had just stood there most of the time, we'd have never known.