One that caught me three or four times was Udemy, where I'd paid for a course heavily discounted on sale meaning to study it at some point later. Much later, I'

One that caught me three or four times was Udemy, where I'd paid for a course heavily discounted on sale meaning to study it at some point later. Much later, I'd go to the course page and find that the author had withdrawn it and Udemy were offering me a refund of the discounted price. All within their rights, but it feels annoying.