How common were forging errors on European blades?
 

Such as blades not being 100% straight, a mistake in the forging process, a dimple in the blade, etc. Swords from the 1600s onwards seems to have survived in the best condition so I guess my question pertains to them specifically.

Common, but increasingly less so as we get closer to modern times. I don't know about modern sword production, but basically everything was made by hand right up into the 20th Century, including forging and grinding of the blades, there will have been templates and gauges to work with, and as we move forward in time the precision of these will have improved immeasurably, but even then the most common gauge would still have been the human MKI Eyeball.

So when you look down a blade you're going to see variations in fuller depth, dimples in the surface and ridges not quite perfectly straight. Also keep in mind that past ideas of 'good enough' don't match with modern expectations.

I think Matt Easton and others have made videos on this that can much better explain it that I can here.