I agree that putting a memo-private key into a field definitely is a mistake and not an accident. It's a kind of mistake I had not heard of before this article.
Putting an active key into a memo field is probably an accident and a worse mistake.
As I recall there is some white-hat bot that scans the blockchain for private-active-keys to put them into a three-day lockup, and a black-hat bot that does the same in order to steal the funds.
When I posted the text of the memo keys in this discussion, @peakd warned against sharing your private keys. So good protection is possible at the interface level.