I made Alice waters 1234 cake and it came out fine, but sagged in the middle. Any idea what I did wrong?

If you repeatedly open the oven door to check on it, the temperature can be thrown off and cause it to sag.
Also, if you overbeat your batter, you can incorporate too much air which I think can cause the same effect.

Oven temp could be off -- on the low side. An underbaked cake will sag in the middle.

Usually that's caused by too much leavening. The gas bubbles get too big and float up to the surface and pop, causing the cake to collapse. If you're at any kind of altitude -- above 1500 feet -- even 1/8th teaspoon can throw off the recipe.

Another possibility -- if the bottom was a bit soggy, too -- the butter was too soft.