I knew you'd be the one who would answer the question. I was surprised at the "two-step" in your first reply.
There was no fancy dance in my first reply. :wink:
Almost every shot is a shot that can and has been missed, even by the best players out there.
There really isn't a single "secret" that explains how a simple straight in shot can be missed. Lack of practice fits most players. But for those who have already developed excellent ball pocketing skills, those who've already put in the practice time needed to develop and tune those skills, it could be a combination of one or more other "secrets" as to why they might miss an easy shot.
I'm talking about focus, confidence, emotional control, stress, etc... I don't care who it is, if the player doesn't have a solid handle on these things, it's likely they'll find themselves missing that occasional easy shot every once in a while, or stringing a bunch of misses together. I've been there. If the mind isn't right, pool is extra tough.
So lack of practice isn't why a good player might miss an easy straight in. That little secret is for less experienced players that haven't quite developed consistent mechanics and aiming skills yet. More often than not, and this is no secret, the reason better players miss easy shots sometimes has more to do with what's cluttering their mind than it has to do with mechanics or specific aiming/ball-pocketing abilities. This game requires focus on every shot, regardless of difficulty. So if there is any secret at all...that's it.