I agree with the theory several others put forth that the picture shows a used (fired) primer that did not get pushed all the way out during sizing/depriming, a step in the reloading process, and the used primer stayed with the case through the next steps in the process of receiving powder and a bullet.
That is, the mark is that of a striker or firing pin that hit the primer and indented it, then was flattened during depriming (e.g., when pushed from the inside). That made it look like a "glob." But the shape is distinctive...
If I (and a few others, above, are right about this) it meant the bullet could not be fired, which is luckily good, but not something you would know while the gun was locked tight. But if you took the round apart, you would find powder but a dead (used, with blackened anvil) primer.
Interesting case, helpful responses, and a good lesson in the fallibility of ammo. For self-defense carry ammo, I'm inclined to believe each round should be inspected before loading in the mags, and only reputable manufacturers should be used!