Ask HN: What is the thing you've built that you regret the most?
I built a lighting system for  to save energy by turning off hallway lights when not in use. The environmental aspect was great and saved hundreds of thousands in electricity. Someone eventually realized that the mesh network I built to connect all the lights together and report usage statistics could also be used to track employees moving throughout the building and catch them taking unauthorized breaks in the stairwell, so that's its main purpose now.I'm a lot more paranoid about privacy these days.