I just had a user tell me his registration key didn't work. It took me quite a while to figure out that he entered the capital letter "O" instead of the number "0".
So I decided that it is best for my registry checker to treat both "O" and "0" as equivalent, so both will work.
I can think of two other possible mixups:
Are these all of them, or are there others?
2 and Z or z. Possibly 9 and g, or 8 and B or 6 and G.