Rhinoceros is an absurdist play by Eugène Ionesco in which the citizens of a small French town transform into rhinoceroses. This premise of the play is the central unrealistic element. Indeed, the transformation of people into rhinoceroses is Ionesco's way of metaphorically exploring issues around conformity and mob mentality. These issues were particularly important in the decades after the rise of Nazism in Germany.
In the play there is no explanation offered as to why people start transforming into rhinoceroses, and there is also no attempt by any of the characters to find any such explanation. One might certainly say that it seems unrealistic that none of the characters seem to question why so many of their fellow citizens are suddenly turning into rhinoceroses. This idea, that something utterly bizarre and illogical should be accepted without question, is a key characteristic of absurdist theater.
There are several instances in the play in which characters break the illusion that the play is real. For example, in act 1, Jean asks Berenger, "Have you seen Ionesco's plays?" Berenger replies, "Unfortunately, no. I've only heard people talk about them." This reference to the playwright is spoken by the characters in the same playwright's play, reminding the audience that what they are watching is only a play. This is another element of the play that can be deemed unrealistic. The reality of the play, from the perspective of the audience, is indeed broken.