Several kinds of characters can create issues.
If you see a weird black-outlined box instead of an actual character, you might have a file with an emojis, characters with accents (cedillas, acute and grave, etc.), or a file written in languages that make use of characters beyond the 26-letter English alphabet. Using “UTF-8” characters can help. Check this UTF-8 Character List for a copy-paste of a wide variety of characters that will display propelrly inst
Note that markdown is sensitive to certain characters, in particular: :, #, *, -, and `. If you have recently made a change involving any of these characters, make sure that this is intended and, if so, that you have properly escaped the character with either a backtick ` or backslash \.
Curvy quotes and curvy apostrophes need to be replaced with straight double quotes and single apostrophes.