Obstructed face printing
An object can block ink from reaching part of a sheet during printing. When that material falls away, it leaves a blank patch. The rest of that printed design may look normal.
Heritage described this on a Series 1985 $10 note. Something blocked part of its face printing, leaving an uninked sliver. The note was graded PCGS 66 PPQ. It sold for $180 on December 30, 2020. That result belongs to that specimen, not every bill with missing ink.
A blank area alone does not tell you what happened. A gutter fold can also leave an unprinted gap. Look at both sides and the paper itself before naming an error.
Doubled and tripled back printing
A doubled print repeats part of the same design. A tripled print adds two extra impressions. The repeated details may be faint or partly overlap. This differs from an offset, which transfers a mirror image from the other side.
One Series 1988A Boston $10 has two extra back impressions. Heritage recorded it as PMG 63, with a small red ink stain along the top edge. The stain is a separate condition detail. It is not what makes the note a printing error. No sale price is quoted here.
Check the whole note
Record which side and which design details are affected. Extra back printing is different from doubled seals or serial numbers. A star or series date alone is not an error.
Added printing can imitate a genuine mistake. These clues help describe a note; they do not authenticate it. Keep the bill as found. Do not wash, press, trim or peel material from it. Ask a qualified banknote examiner about an uncertain find.
Keep the complete assigned grade with a specimen record. Read what banknote grades mean, or return to the short $10 bill error list.