Sometimes you'll declare a charge thinking that it can be completed, but then discover that proximity of other units, terrain or just whimsical and unkind geometry (by which I mean 'it won't fit') prevents the charge from being completed. In such circumstances, I find it best to treat the charge as having failed, using the rules given earlier. Alternatively, you and your opponent might agree that the sensible thing to do is declare that the charge never actually happened.