Ok I've been using compasses for a while and I understand the principles on how they work (have to teach it to 11-16 year olds...) but I'm stumped at this one.
Usually use Silva compasses as they're basic and fairly robust, we give them to the kids for DoE to use. I went out last weekend and there were 3 compasses in the group, the kids had 2 and I had 1, all identicle and made by silva. I put my compass in my top pocket (nothing else in there), next time I took it out to check a bearing, walked about 200 meters and the compass swithced so that north points almost to south and vica verca. Only happeded with this compass and its still like it, so a permenant change.