VDMONA is simple way of remembering the more common compass errors
Variation
Deviation
Oscillation
Magnetic dip
Northerly Southey turning errors
Acceleration Deceleration errors
Compass swings are used to help compensate for installation errors , Some freight can make your compass do interesting things . Flying low level the compass will spin like a spinning top over some mineralization areas ( large Gold bearing quartz deposits) .
My favourite AME had a sign over his desk ;
Questions answered fifty bucks
correct answers one hundred
dumb looks are still free ,