For my 3D audio project I need an affordable way to make plastic cases. I felt like injection molding services are way overpriced, so I decided to make the molds in-house. Turns out, CNC milling is overpriced, too. As are 5 axis CNC mills. So in the end, we built our own CNC machine.
And like these things always go, I found an EMI issue with my power supply and a USB compliance bug in the off-the-shelf stepper control board. But it all turned out OK in the end so we now have the first mold tool that was designed and machined fully in-house. And I learned so much about tool paths and drill bits. Plus it feels like now that everyone has experienced hands-on how stuff is milled, my team got a lot better at designing things for cheap manufacturing.