Getting everything perfectly square and level. No matter how careful you are, there always seems to be something off (it happens to the pros too, so I don't feel too bad!) Of course, I made things more difficult by choosing a 1/16" reveal which left no room for error. This was my biggest hesitation about building our own cabinets, and I was prepared to settle for 90% perfect to get this look. I'd say we're at 95%, so I'm happy :)