Not sure if it’s what you’re looking for, but I’d say the mystery of the dead bear cub in central park 10 years ago. This was minor news back then and basically was forgotten about, until RFK Jr. (former third party presidential candidate) admitted to being the person behind it, confessing it to a confused Roseanne Barr.
FWIW this isn’t always true. A few months ago, I needed to add an email to my Zelle account on Chase, and had to call them. I initiated the call and they did issue a text message verification to the phone number in my account while on the phone to confirm.