A letter D just below the date is the mint mark for Denver.It is called a mintmark, and it signifies where the coin was minted.
If you mean it does not have a "D" or "S" mintmark . It was made at the Philadelphia Mint. No U.S. one cent coin has ever had a "P" mintmark.
On average, 3 cents or so.
Average value is 3 to 10 cents.
If you mean a "D" under the date, it's not an error, it's the Denver mintmark were the coin was made.
A 1941 penny without a mintmark means it was minted in Philidelphia.
The Lincoln Penny was produced in both Philadelphia and Denver in 1956 (the latter have a "D" mintmark below the date).
On a US coin, an S mintmark means it was minted in the San Francisco Mint.If it has a D it was made in Denver.If it has no mint mark letter it was made in Philadelphia.
There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
It depends on the mintmark and condition. A 1928 penny with no mintmark (thus minted in Philadelphia) in not that great of condition might only be worth $.10, however if it has a D (Denver) or S (San Fransisco) mintmark it could be worth a dollar or so in the same grade. If you have a perfect example it can range anywhere from $90-3,000 depending on the mintmark, but those are only for the absolute best coins, if you picked your coin out of circulation on average it would be worth $.10-$.20 with no mintmark and I'd estimate about $1-2 if it has an S or D mintmark.
A Lincoln cent without a mintmark was struck at the Philadelphia Mint. The Mint does not use "P" mintmarks on penny's.
If the coin has the mintmark D then it would be below the date on wheat pennies, memorial cents, and the current design. The D means it was made at the Denver mint facility.