They happen when you have at least two auto-reply systems set to respond to every single email that somehow start messaging each other.
My absolute favorite was the one that literally said “hodilay”. Typo included, capitalization, or any other words, not.

Hoo boy, have I got some PTSD from Old Job about out of office autoreplies. Exboss was such a stickler for them and actually enforced her expectations as official policy. Meaning if you didn’t do it to her exact specs, she’d call you back to the office to do it (which no one did) and read you the riot act afterwards while threatening to write you up for insubordination. She demanded them any time that we were away from our desk for longer than 30 minutes and for anything other than a meeting. So training in the conference room down the hall, a work lunch with teammates, leaving an hour early for an appointment, arriving late for an appointment, even working from home, all required OOO alerts.
Get the time and date right for when the automatic emails start - you could choose the the moment that you actually leave the office, or some cheeky people choose an hour or two before they leave, saying that they are busy handing over or wrapping up to deal with customers or other colleagues. They are still able to check the emails they do get anyway, to reply properly to if they want to.