Here's one that that's the bane of letterers and colourists everywhere, because they usually get lumbered with the job of fixing it…
Pencillers: learn the difference between Bleed, Trim and Live areas.
Honestly, it's not difficult -- half the time, I see the measurements printed on your artboard! Don't take it the wrong way if you don't know what these mean, though… as I said in my first piece on this subject, all this stuff should have been explained to you long before you got to the stage of making a professional comic page.
Most pre-ruled comic board will look something like this.
(By the way, the rules are printed in non-repro blue to make it easy to separate them from the pencils. If you scan the pencils in grayscale rather than RGB, someone has to manually remove them with the eraser tool!)
Perhaps it's better to back up a bit, and explain why these measurements are important…
A finished, printed US comic page is (more or less) 175mm x 267mm. This can vary a bit by publisher, and some Print On Demand (POD) services use a different page size, but this is pretty much close enough to the measurement of most published comics.
What this means is that the finished artwork will have been printed as spreads* onto pages that are oversized, cut down to the final size on a large guillotine, then mechanically folded and stapled to produce the finished comic. Perfect binding is a slightly different process, but makes no difference to the basic principles.
Here's the thing: if you've ever seen paper going through a press, it goes at a hell of a speed. Plus, it's having ink applied to it, which makes it wet, meaning that it can stretch slightly. All of which means that it moves about on the press while it's being printed.
If you -- the artist -- want a panel's artwork to extend right to the very edge of the page, then this means that you actually need it to bleed off. Because the guillotining process cannot be 100% accurate, you have to draw your artwork larger than the finished size and allow a margin of error.
This is why you have a Bleed and a Trim area marked on your artboard. The Trim area is completely irrelevant to the artist, to be honest, other than to understand that you can never, ever, end your artwork on that line, because it will either get chopped off in the finishing process, or you'll get left with a nasty white gap at the edge.
Consequently, anything that doesn't bleed off the edge of the page needs to allow an additional margin to ensure that it doesn't get cut off if the guillotine cuts in too far (once again, due to that movement on the press). That means panel borders, all the important parts of the action, especially characters' heads, any signage that needs to be readable, and all the lettering have to go inside this inner area, referred to as the Live or Safe Type area:
This last, the Live area, can vary widely dependent upon publishers' and printers' specs and I would urge any penciller taking on comic work that will be printed to check these three measurements with the editorial or production team.
Asking questions doesn't make you look stupid -- getting it right makes you look professional.
As ever, feedback or questions are most welcome in either the comments section here, or on whichever forum you saw me plugging this blog!
*Technically, as "printers' pairs" where, in a 32-page book, Pg1 pairs with Pg32, Pg2 with Pg31, and so on. This is all done automatically these days, so don't give it another thought.