When Cobb and Saito both wake up in Saito's apartment after Cobb has attempted to extract Saito's business secrets, the viewer expects that they have both re-entered reality. In fact, in an example of situational irony, Cobb, Saito, Arthur, and Nash are revealed still to be dreaming, and are actually on a moving train in reality.
Robert Fischer's dream (dramatic irony)
The audience knows that Robert Fischer is dreaming, but Robert Fischer himself does not. Until Cobb tells Robert explicitly that he is dreaming in order to convince him to go deeper into the dream, dramatic irony is structuring our perception of Robert.
Arthur's kiss (situational irony)
When Arthur tells Ariadne that kissing him will help divert the attention of the projections around them, the audience expects that this will work, given that Arthur is a dream-sharing expert. Instead, in an example of situational irony, it does not work, being merely a pretext for Arthur to get Ariadne to kiss him.
When Cobb and his team first descend into Robert Fischer's mind, the audience expects that they will easily be able to navigate the dream-world that they themselves have created. Instead, situational irony occurs when a massive train plows through the dream and the projections in Robert's dream turn suddenly violent.
Saito's rescue (dramatic irony)
When Cobb descends into limbo in order to save Saito, the audience has seen a glimpse of this exchange already, inserted into the opening sequence of the film. Given that the audience knows that Cobb is trying to get Saito to wake up but Saito does not, this is an example of dramatic irony.
Inception Questions and Answers
The Question and Answer section for Inception is a great
resource to ask questions, find answers, and discuss the novel.