Various MUAs have failed this over the years. RFC 5322 is dated 2008. Plenty of users moved to HTML payloads inside MIME attachments because of the mangling that would happen otherwise. Many MUAs would prepend a > before the word From at the beginning of a line so that it wouldn't cause problems for recipients.

Some that I used that I remember such problems with include the MUA inside Netscape Navigator and mutt.