Have you read all the stuff year after year about why planter spacing is important and you think it's just hype? Nothing beats seeing what someone is trying to explain. Take a walk into one of your cornfields and unless you did a 100% perfect job of planting, you can probably find an example of a stalk that was planted too close to a neighbor, that emerged late, or maybe both.
What you likely will find is a skinny stalk with limited stalk diameter. There may be an ear, but it's likely a nubbin. It may or may not have kernels, and no, it's not 2012. It just had too much competition from neighbors, either because it was crowded or emerged late.
So what's the big deal? One field we checked recently was on the borderline of being too dry, and also suffering from N deficiency. That stalk which isn't going to produce a viable ear is now a weed. It is taking up moisture and nutrients, especially nitrogen, needed by its neighbors to try to finish as many kernels as possible on the ears on those stalks.
Even if you do a good job of planting and score a standard deviation of 2.0 or less, you will have some skips and doubles. The skips mean perhaps fewer plants per acre. But if the average population comes out right because the skips were offset by doubles, it's the doubles that may or may not become a problem. One may produce an ear, the other not. Sometimes neither produce ears, or only small ears. Occasionally, if there is ample moisture and enough nutrients, both produce decent ears.
If you see those doubles or plants with no ears this fall, it might be time to look at your planter again before next spring make sure every unit is working as you want it to work.