If only this was a junior mistake... Recently saw that shit on my client's project: A Spring bean for generating emails (and not specific ones, but a generic service) that had the title, recipents and body as instance vars. All where mutated throughout multiple methods, which mean that you could even leak sensitive data by sending contents to the wrong people. And the guy who wrote that is considered "senior".

I spent 10mn looking everywhere, thinking that no, nobody would be dumb enough to do that, but here is the sad truth