I've had a custom house built by a very well-respected GC in my region. Despite not doing any of the actual labor, I'm doing an incredible amount of work. Since it's a totally custom house, I spent all weekend for five or so months going to supplier offices to pick out materials, designing tile work & shelving, tweaking layouts, etc. Since the project has taken so long, I've had to deal with suppliers running out of materials and having to redesign a significant portion of the house. I even hired a professional designer to help with ordering and negotiating.

I'm at the house after every trade leaves to ensure things are done correctly. I've had to ask for dozens of things to be redone because it was just wrong. Doors were the wrong size (and installed anyway, despite having written on the door the correct size), the wrong color, outlets were missed, the stairs had to be moved (!!!), the tub was backwards, etc.

Additionally, even though it's a custom house, I already regret some design decisions, but it's impossible to fix them now.