Theory #8: Cobb began to dream after he met Yusuf.

What if everything from when Cobb met Yusuf was just a dream? Think about it, Cobb went down to the basement with Yusuf and saw all the old people who were dreaming. He then decided to test out Yusuf’s special sedative. This could be the last moment he was truly awake. When he woke from the sedative, he tried to use his totem but was interrupted by Saito in the washroom. Cobb wasn’t able to tell if he was in a dream or not. From this point, we never saw the top fall again. This meant that, technically, we could never know if Cobb awoke from that dream in Yusuf’s basement.

(taken and rephrased minimally from Dream A Little Bigger: A Guide to Understanding Inception: http://wegotthiscovered.com/2010/07/20/dream-a-little-bigger-a-guide-to-understanding-inception/)