Be careful what you copy: Invisibly inserting usernames into text with Zero-Width Characters
How?
The exact steps and logic are detailed below but to put it simply; the username string would be converted into its binary form and then the binary would be converted into a series of zero-width characters representing each binary digit. The zero-width string could then be invisibly inserted into the text. If said text was posted elsewhere the zero-width character string could be extracted and the process reversed to figure out the username of the person who copied it!