Oh no! I forgot to buy that!

As starter chefs, we can hardly be expected to have everything we need in the kitchen all the time, and at our fingertips. I found this great list from Vegetarian Times that helps you substitute ingredients that you are likely to have in your kitchen if you can't find or are out of the ingredient that the recipe calls for:
If you don't have...

Atta (chapati flour)
Use 1/2 cup all-purpose unbleached flour plus 1/2 cup sifted whole-wheat flour

Chana dal
Use Split yellow peas

Curry powder
Use Mixture of ground ginger, cumin, coriander, fenugreek, turmeric and fennel

Garam masala
Use Mixture of 1 tsp. cardamom seeds, 1 Tbs. cumin seed, 1 Tbs. coriander seed, 2 tsp. black peppercorns, 1 tsp. cinnamon, 1 tsp. cloves, and 1 tsp. nutmeg

Jaggery (coarse palm sugar)
Use Date sugar or brown sugar

Toor dal, urad dal, mung dal
Use Red lentils

This list just shows the Indian substitutions, but the site is comprehensive and has subs for Asian, Mexican, and Mediterranean, too.