I'd expect there to be 2 positions between the "e" and the "t" that the cursor would stop at so I can either insert text with the formatting of "one" or "two"
A string that ends in *bold!*
//
AB
You can then pretty quickly see what goes wrong here:
A string that have *_italic bold_ and bold!*
///
ABC
I think you could turn any of these into interesting box forms like one of the links in this thread that has two colored boxes, and you could have text somewhere (like between A & B in the second example here) and delete that text, and suddenly not be able to get a cursor back there, short of laying some text down and reformatting it.
Markdown, and other markup/syntax based inputs, make this trivially obvious, to a large degree. (Some ambiguities in the syntax can cause issues, but those seem to be rarer than issues w/ Slacks WYSIWYG editor.)