My Wife is currently fuming in the other room because she sent the wrong signup link to several business partners, because someone editing that the link did so in an HTML/Rich Text editor, and ended up editing the text of the link, not the source. So she sees the correct link in her editor/email, but in fact she’s sending someone else’s affiliate link.

I pretty much only use plain text email, and this is one reason - if it’s not spam or phishing, it’s simple ignorance.