An almost random, un-retouched panel scanned from the Zot #31...
Notice anything... "off"? Or seemingly out of place? How about I zoom in on a tighter cropping for you...
There are three white circles in the black areas that don't appear to belong there. See them? One in the lower left corner, one in the upper right corner, and one on the left side up towards the top. Those weren't put there by artist Scott McCloud.
Those are actually accidents that occurred during the printing process.
Traditionally, printing involved making a mirror version of the image you wanted to produce, carving it in relief so that all the areas that were to be printed were raised, covering it with ink, and then pressing that down onto a piece of paper. Basically, the same process as a rubber stamp. For professional printing, they generally used metal plates, though, as rubber didn't have a tendency to hold up very well on large print runs that you might find in, say, comic books.
In the 1980s, other materials were found to be used that were more flexible than the old metal printing plates but still durable enough to remain viable for larger print runs. The process was called flexographic printing, or flexography. The major benefit of this was that, due to the type of materials used, printers could use less viscous (i.e. thinner) inks than they could with metal plates. The thinner inks dried MUCH faster than before, thus making comic book production much quicker and cheaper. That was one of the reasons for the rise in the number of self-published comics during that time-period -- it was simply more affordable than it had been.
The down side to using thinner ink, though, was that it was more likely to allow air bubbles to get trapped within it. The problem with having air bubbles in your ink is that, when the ink is pressed onto the paper, you don't get any ink coverage in the spots where air bubbles have formed. Which means that the large black areas that are supposed to have lots of ink coverage get little white circles in them like you can see in the example above. The problem tended to be more prevalent in indie comics, but I recall seeing at least a few Marvel issues that had similar problems around the same time.
Of course, printing technology has changed radically in the past 20 years or so, and you don't see much of this any more. But it is a curious artifact of a specific period of time in comic history and, as I noticed it while riffling through some older comics, I thought I'd elaborate for anyone else who might come across the problem and wonder where the heck those circles came from.
This has been your comic book production blog post of the day.