When the mint mark is not added to the master or working die. These are not to be confused with mint marks removed from the die when a mint employee was filing away clash marks or trying to prolong the life of the die.
1922 No D Lincoln Cent is not an example of an omitted mint mark, it was on the die but accidentally removed. Struck through grease filled die is also not an omitted mint mark.
Omitted mint marks are mostly known for proofs without the S, but there is a 1982 Roosevelt Dime without a P mint mark, but Lincoln Cents never had a P mint mark until 2017. So no mint mark cents are normal and where minted at the Philadelphia Mint.