Claire Bolton, 12 October 2020
Occasionally we encounter type letters that have fallen from the locked-up type in its forme, landed on the inked text, and been printed, thereby leaving an impression in the printed page. They are usually caused by the printers’ error of poor lock-up. When the forme of type letters was not locked up tightly enough for printing the letters could be pulled up by the sticky ink during the inking process.
The presence of a piece of type on the set page also stops the other nearby letters printing onto the sheet – the fallen type and its surrounding ‘halo’ of unprinted letters is usually quite noticeable on the page. It is something that the printer would have noticed quickly, perhaps after just one printed copy, and would then stop printing to repair the damage to the forme by replacing any crushed letters around the fallen letter. It is unlikely that the same fallen type letter appears in more than one copy, unless the impression was on a woodblock, when the printer often carried on printing regardless.
These impressions are rare. Those found and recorded to date in incunabula can be seen in MEI (Material Evidence in Incunabula) under Fallen Type. [To find out where the fallen type examples have been found go to cerl.org and then click on the MEI logo and enter “data.copyType:n” into the search space.]
These are not just curiosities because they bring with them information about printing and type in the earliest period. It is possible to measure the type height and width, and occasionally type depth of the fallen letter. Type heights in the examples found so far vary from c.20mm to 27mm height. Some fallen type letters have flat feet, some have sloping feet, some have square shoulders and others sloping shoulders, and some have a hole drilled though them

The surest way to find an example is to study the printed leaves of a book closely, one by one, not just flicking through.
See another example at the website of the Association of European Printing Museums (AEPM); https://www.aepm.eu/wp-content/uploads/2012/09/MicgInv0472_211b.jpg