Forgers, including disgruntled or terminated employees, know what self-correcting type looks like and can identify and alter these manually issued checks with ordinary transparent tape. They simply lay the tape over the letters to be removed, rub the tape lightly with a pen or pencil, and lift off the tape. The typed letters are on the tape, not on the check. A person with only moderate finger dexterity can clean off a check completely in about 90 seconds. He or she then types in a pseudo payee name or new dollar amount and negotiates the check, with the original signature.
Some companies issue manual checks by hand, literally.
A ballpoint pen is the tool of choice, with information handwritten on the check. This is a dangerous practice because ink from a pen, including so-called permanent ink, can easily be dissolved with acetone without destroying the check. When issuing manual checks, use a typewriter with a fabric ribbon, not a polymer ribbon. Fabric ribbons are available for almost every make of typewriter through the catalog department at major office supply stores. I strongly recommend a single-strike ribbon, because it ensures that the maximum amount of ink is driven into the wood fibers of the paper.