Easily confused symbols
Some symbols are hard to distinguish, or are easily confused for each other, particularly when handwritten.
When 1 is written with a twiddle at the top rather than just as a straight line, it could look like a 7. Or, a hastily-written 7 could look like this kind of 1.
A common way of resolving this is to draw a line across the middle of the 7.
m, μ, u and n can all run into each other, particularly when written in cursive.
A famous example where these combine to produce something hard to read is the word 'minimum':