Or its a simple signifier that the author was human, and that a real person is trying to convince you of something. I've experimented with putting minor grammar mistakes into my work of the sort that would be frowned upon, but are not strictly invalid. The existence of any kind of mistake makes the work sound "human".

Don't know about that as a general rule, since spam messages have had typos and mistakes in them since forever, and its precisely what marks them as not trustworthy.

Who said signalling would be limited to just 1 thing at a time?