Photos photos photos during the build. Of all the structural work, frames, electricals, plumbing, waterproofing, everything.
We currently have an issue with a water leak in our patio ceiling. The builders are stumped as to how the water is getting in. Fortunately I had some random photos of the waterproofing stage (not actually deliberately of the waterproofing, more views from the room at the time) but they proved invaluable to the builder. If I had known, of course I would have taken more detailed photos but hindsight is always a handy thing. Likewise, DH took lots of photos of plumbing and electricals, which have been referred to numerous times since we moved in (eg. when we had no hot water at hand-over and the builder wanted to check the cross overs and when DH has installed things on the walls it has helped to remember where electrical wiring is to be careful).
So a reminder that you can't take enough photos (although I am sure we all do), but even of the boring stuff as you never know if you will need it in the future.