Various unicode control characters do not show up. The list of unicode characters that can be used in an obnoxious manner on internet forums is so long that it is almost impossible to do right.
My go-to is the Cyrillic Millions sign, which is intended for use in bi-directional text. For an example, copy the line below and paste it into a browser text area and try typing off the end:
"This is ".ti od ew woh
I can't even copy it, let alone type at the end of it. When i reach a little less than halfway through, it breaks off and selects the other half. This is interesting. I am guessing, there is a hidden unicode character that breaks of the selection.
Good question. The one I posted above was an accident; I don't know how I did it.