I distinctly remember a scene in Inception when Cobb is testing Yusuf's custom made super sedative. He then abruptly wakes up and goes to the bathroom where he proceeds to splash his face with water and spin his totem. However, before the top can reveal whether he is in a dream or not, it is interrupted and knocked off the counter. Cobb then sees Mal in the reflection of the mirror. Surely Nolan made the scene in order to help the audience understand how Cobb is starting to have trouble differentiating what's real from what isn't. But is there any definitive evidence to prove that the rest of the movie is not just part of a sedated dream he never escaped?
In order for this theory to make sense, we need to make a number of fundamental assumptions. Since we see Cobb using the top in the earlier scene with Ariadne (it topples) we have to assume that this is reality. Shortly after he meets her, she takes him into a dream world of her making, with highly complex architecture and filled with imagery from her imagination, not his.
In the scene you've mentioned, he does indeed fail to use his totem correctly and we could, theoretically still be within his dream. The question then is "whose dream is he dreaming?". If the remainder of the film is Cobb's own dream then that beggars the question of who is architecting everything for him.
Are we genuinely to believe that Cobb has sufficient imagination to independently control the actions of a dozen actors, several of whom are working at cross-purposes, at differing speeds within multiple layers of the dream world? After Cobb goes to Limbo, is he still controlling the actors on the higher levels, all of whom are moving thousands of times faster that he is?
Based on what we know of how dreams (in the inception-verse) work, this theory simply can't be true.