As a primary teacher I teach maths and when teaching co-ordinates, I put across the (x,y) concept by saying, "Go along the corridor and up the stairs", but am horrified to discover that, even though I aced my TA map-reading exam, I have been misreading maps all my life. Effectively, you give the y axis first! This is bound to result, one brain-fuzzed day, in me doing a Wrong-way Corrigan and flying off into the middle of Atlantic, looking for Birmingham.
I have started a thread on my newly-induced co-ordinate neurosis on the TES teacher's forum, where I post as "crucible".
UPDATE: Well, that is a relief. It seems from talking with various teachers on the TES that Ordnance Survey maps do work in exactly the way I have used them all these years, as this explanation shows, but marine maps (and aerial charts too?) use the "world geodetic survey 1984" system, referred to as WGS84, which is the datum used for GPS. It seems I am not mad and that others share my confusion and surprise at the conceptual mismatch.