We see the final scene before everything else (Dramatic Irony)
The structure of the film, in which the color scenes go in reverse chronological order, creates many instances of dramatic irony. The most significant one is the fact that we see Leonard kill Teddy before we get any background information about their relationship or who they are. Thus, we spend the entire film knowing Teddy's fate before he does. The film fills in the blanks gradually until we see what leads to the moment of Teddy's death.
Don't talk on the phone (Situational Irony)
Throughout the film, we see Leonard on the phone with someone, telling them various details about his biography and the story of Sammy Jankis. At some point in the phone conversation, Leonard looks in the mirror where he spots a tattoo that he got to warn himself not to talk on the phone. This is an ironic twist, in that Leonard realizes he shouldn't be talking on the phone, after being on the phone for an extended period of time. It is also ironic because it is a note from himself that brings him to this realization.
Dodd in the closet (Situational Irony)
When Leonard finds Dodd tied up in the closet of a motel room, he takes duct tape off of Dodd's mouth and asks who did this to him. In an ironic twist, Dodd informs Leonard that he did this to him. Leonard doesn't even remember that he tied Dodd up and prevented him from talking. There is a darkly comic irony to a kidnapper not realizing he did the kidnapping.
Teddy's initials are "J.G." (Situational Irony)
By the end, we learn that Teddy has manipulated Leonard into thinking that anyone whose initials are "J.G." is his wife's assailant, as Leonard has forgotten that he already killed the assailant a long time ago. Teddy has been manipulating Leonard into killing people for his own gain. The irony in this, however, is that Teddy's own initials are "John G."—making him an easy target for the forgetful Leonard.
