100K black vs. rich black
100K black (only black ink, no cyan, magenta or yellow) is the simplest and most reliable choice for text and thin lines: no risk of misregistration between colors. On large areas, though, it can look flat or slightly gray. Rich black (a combination of K with percentages of C, M, Y) produces a deeper, denser black on large solid areas — but introduces the risk of print misregistration, and should never be used on small text, where even a slight shift between colors creates a visible halo.
What overprint is
By default, an element printed on top of another "knocks out" the color underneath, removing it in the overlap area. Overprint reverses this behavior: the top element prints on top of the underlying color instead of removing it. It's a precise technical function, not a "safety" setting to turn on always.
Why overprint can solve — or create — a problem
On small black text or thin lines over a colored background, overprint prevents a minimal print misregistration from leaving a visible white gap along the edges of the text (a real risk with standard knockout). Applied by mistake to large elements meant to appear as solid color, though, overprint can produce an unwanted color mix that wasn't intended in the design.
It's not a universal rule
There's no overprint setting that's always correct: it depends on the type of press (traditional offset handles overprint one way, many digital presses ignore it or interpret it differently) and on the printer's specific RIP settings. The same choice that's correct on one press can be wrong on another.
A practical check
In Adobe Acrobat Pro, Output Preview includes a "Simulate Overprinting" option, which lets you preview the real effect of overprint before sending the file to print — the only reliable way to catch a setting applied by mistake.
Common mistakes to avoid
Unintentionally applying overprint to an entire color field, often from a setting inherited from a layer or group. Using rich black on small text, where the misregistration risk is more visible. Assuming digital print behaves like offset when it comes to overprint: often it doesn't.