I think my favorite part of Lean Startup may be about The Five Whys. Obviously everything about minimum viable products, metrics, small batches, and feedback loops is great. But The Five Whys is profound. It's adapted from Toyota and is used to get to the root causes of problems.
Excerpt:

When confronted with a problem, have you ever stopped to ask why five times? It is difficult to do even though it sounds easy. For example, suppose a machine stopped functioning:

1. Why did the machine stop? (There was an overload and the fuse blew.)
2. Why was there an overload? (The bearing was not sufficiently lubricated.)
3. Why was is not lubricated sufficiently? (The lubrication pump was not pumping sufficiently.)
4. Why was it not pumping sufficiently? (The shaft of the pump was worn and rattling.)
5. Why was the shaft worn out? (There was no strainer attached and metal scrap go in.)

Repeating why five times can help uncover the root problem and correct it.

• Be tolerant of all mistakes the first time.
• Never allow the same mistake to be made twice.