There are always places that get missed or left behind. Sometimes it's due to a lack of time; sometimes because of a lack of knowledge. Whatever the reason, and despite the fact that it's both normal and unavoidable, I almost always feel like I've failed in some way.
Of course, it's not a failure at all; there is so much more than we can ever see. Sometimes - the best times - there's so much in a given area that it's easy to plan an entire trip. Other times, there's not quite enough. In those cases, it's always nice when a few of those places can be enjoyed On The Way to another destination or trip.