Many experience walkers are aware of this phenomina when in the mountains. I had an interesting experience many years ago when I was leading a group of walkers on Carnedd Dyfed. We where in a 'White Out' and it is then you need to navigate by compass. I sensed I was leading the group onto a steep slope. I noticed the compass needle was erratic and moved the group to the left of the slope. It wasn't until I got back to the youth hostel when I was told that the mountain I was on did have a magnetic anomaly. If I had continued walking to the right of the slope I would have taken the whole group over the precipice.   

It is only when you are in conditions which require you to use a compass when you become aware of this problem. It only happens in mountainous areas i.e granite . You should always ask 'Youth Hostel' or mountain rescue people whether it occurs in the area you are walking. 

It makes me wonder how many accidenets have been caused by this, especially in 'White Outs'. I remember asking the Ordnance survey about putting a a square around areas on a map where this could occur. They believe it would be  too expensive. 

Anyway, always be aware of this when walking in the mountains. Has anyone else experienced this I  would very much like to know.