I had a habit of hiding my face in little hard to spot places of sites I worked on in very low contrast. Over my five years there I worked on probably hundreds of sites.
A couple weeks after I'd left for my new gig, I get a message from a coworker that a different coworker had pushed up an untested change that broke the navigation on one of their biggest clients. The HTML had become malformed in such a way as to make the entire page a repeating pattern of my very low contrast face, which had been hiding under the navigation.
The client thought it was some sort of hack. My friend and coworker knew what I'd been up to and explained to the team. I have no idea what was said to the client.
A couple weeks later it happened again with a different site. Same developer pushed bad code and my face hidden under the contents of the page were revealed.
They ended up going on an Easter Egg hunt and removed most of my hidden faces.
It's not something I would do these days as it's very unprofessional but I still get a little smile thinking about it.