Decades ago I had an idea for a circuit which would remove Macrovision copy protection from an analog video signal. IIRC it was based around a sync separator and I was positive it would work. So I ordered the parts from Mouser, wired everything up on perfboard, connected to my DVD player and... nothing. It was at that point that I realized I did not have the equipment necessary (e.g. a digital storage oscilloscope) to determine why it was failing.
The lesson was to plan for failure and before starting the project make sure you have the tools necessary to debug any issues. (In particular any hardware project.)
For example, I considered trying to build a portable Wii (Wiiboy). However, I could easily wind up with the same problem - something doesn't work and I don't have the tools to debug the problem. (Especially since a big part of the project is to physically cut the Wii PCB, so there's a very real risk of something going wrong.)