I had an encounter once in flight when the autoflight system captured an ILS, both LOC and GS, at FL350 and started to climb with quite a rate. Very unpleasant as we were just put on heading and told to descent. During that button push exercise my colleague inadvertently pressed on the APP button which armed the approach mode. Since then i always tune a VOR instead of an ILS during cruise to make sure the APP mode won't work.