Feedback from the very first project I made as a hobbyist, and was proud enough to show online - notably written in Java:
"Wow dude, have you never heard of OOP?"

And that was the day I heard about OOP.

But I think the more interesting ones are always, "things I broke as a junior professional" - such as, getting an angry call from the company's live-ops team asking why they were getting 10,000 failed authorization attempts every hour from the server we were running. Or "that time we accidentally filled the HDD on a bunch of other colleagues computers overnight", or "that time we accidentally broke those colleagues computers and had to call IT to re-install the OS; because the script we wrote to try to fix the overfilled HDDs deleted everything else too"...