In my last year of high school a pushcart driver came to our school daily and sold cold Cokes and delicious warm Chelsea buns to the hungry students at break time: no cafeteria in Zimbabwe high schools, unfortunately.
After a week or so of this, my friend Aaron pulled me off to one side and told me that I shouldn't buy from the guy at all, explaining that I should stand behind the seller and see what he did before he sold the Cokes and buns. So I did so: I saw that the seller had a few large pimples on the back of his neck, which he would surreptitiously squeeze the pus out of and then covertly wipe his fingers on the bun before he handed it to the buyer.
I haven't eaten a Chelsea bun since.