I did this once with the deliberate intention that be traceable. Holding down a few keys while clicking on a specific small set of pixels in an application we were developing for a customer would produce a specific message that was attributable to me personally. I did this sort of thing in most things I worked on back then, partially as a "calling card" and partially as a means of being able to prove my involvement in a given product if the need ever arose. This saved our bacon when a sales rep sent an "evaluation" copy of the nearly completed product to the customer, without authorization, that consisted of our entire source tree. (This was the 1990s and we were running MS-DOS, no file security)

Suddenly the customer tells us they have "repurposed" an "internally developed program" and they cancel our contract. The sales rep was upset that he would not be getting a commission on the sale and mentioned that he had "even sent them a demo", which led to the discovery of what had happened. My boss and I went to the customer's site to "finalize the paperwork", and when we were shown the application (which had extremely minor UI changes) I checked if the hidden click spot still worked, and it did! The customer was horrified. My boss was outraged.

We ended up getting paid the full contract amount plus a large bonus in exchange for agreeing not to sue. The sales rep decided to leave voluntarily rather than be fired.