I'm using a Unicode font that contains characters from the Mathematical Alphanumerical Symbols block (U+1D40X) to create formula sheets for math, physics and chemistry. This block contains italicized and bolded letters to use as variables -- and before someone suggests it, the design of these characters is different from the usual italic characters, so you can see what's a variable.
When I reload the file, a subset of the capital letters (U+1D438 through U+1D44D, E through Z) all change to the character five behind it in the order of the characters in the font, reglardless of which block the destination character is from (some are from the user-defined blocks, so there's no way that there are equivalences built into Unicode for these characters, and Illustrator doesn't mark any of these characters as having OpenType alternates). This means I have to change every capital letter to the correct character at random intervals when I edit or print the file. Illustrator helpfully suggests the old, correct character as an alternate, but I still have to change each one individually.