Situational irony is an occurrence where the intended result turns out contradictory. This result can range from the humorous to the tragic.
An example of situational irony is a scene in a movie where a man is about to shoot someone, however, when the noise of the gun being fired occurs, instead of the bullet hitting the victim, the shooter himself is shot by someone else.
Another example would be a woman who is apprehensive about attending a wedding due to being single, she however goes and there meets her future husband.
In literature, William Shakespeare's Romeo and Juliet provides an example of tragic situational irony. Juliet takes a drug to fake her death, Romeo however takes poison as he believes Juliet to be dead, when she awakens from her self-induced coma, she finds Romeo's body and thus kills herself for real.