Nowhere near as impressive, but at work a few years ago, we needed an equipment management and booking system. None of the systems available at the time fitted all our requirements (namely that any bookings made had a risk assessment uploaded, and were only allowed to proceed *if* the risk assessment was approved). It also needed to integrate with our existing inventory system.
So, we built the system in house.
Students actually used the booking system directly (hence the need to enforce the risk assessment requirement). One day, a student complained that the booking site wasn't working properly. I asked him what he was doing, and he went through the procedure he was following. I said he wasn't using it correctly. He said he was, as he had always used the site that way, and I was wrong. I said I wasn't wrong. I designed and built the site, so knew *exactly* how it should operate.