If you reverse a route which has an alternate specified then the original alternate if appended to the new reversed route. This in itself is not a major problem unless (as I did), whilst in flight, you then cross the track from your destination to the original alternate (which will usually be well behind you) - at this point the wrong leg can become active and guidance is then given to your original alternate instead of the destination. If flying IMC this can be disorienting until you realise what has happened.