Since computing began with alpha-numerics there's been a problem.
Certain numbers and letters all look alike.
For example, the favourite "hilarious" trick to do to your mates when I was a young programmer, was to do a search and replace of all the zeros in someones code for the letter "O".
This of course, made the code fail to compile. But the real fun bit was watching your friend manually find and replace all the invalid letter "O"s one by one manually (because if you searched and replaced the other way, you'd just have the reverse problem).
In many (read most) fonts they look exactly the same. In fact look at these now, they may well look exactly the same on your computer: O, 0.
Of course the letters "I", "L" and the number "1" also have their own confusing variants.
I can't begin to tell you how often I've thought a 1 was an I or an l (FYI: number 1, uppercase letter (eye) "I" and lower case letter (el) "L").
In certain fancy fonts even more letters and characters can look alike. It is a fricken' nightmare if those things are included in a written password that, for some reason, can't be copied and pasted.
Couple this with the fact that most non-computer savvy folks assume that upper-case and lower-case letters are the same thing. To a computer they aren't.
I've urged you before to be careful with how you create and record your passwords but now I urge you to take the extra step of being especially clear with the number and characters mentioned here.
I.e. wherever you store your passwords, make sure you include a written memo about which characters are used, if there is any way they can be misconstrued, and whether or not they are indeed upper or lower case.