I didn't put any myself, but one of our deprecated app had a classic konami code easter egg. A T-rex was crossing your screen when entering the code. The funny story with this was that someone printed a report (there wasn't any feature for printing, he only printed the page from the browser) and the t-rex printed on the page. My boss had some explaining to do and we finally set it to display only for a specific user. In the new version of the app, it displays random GIFs picked by some dev when they resigned. It's now a tradition when you resign (in good term)