The literary device that Gwendolen uses when she says, "I think there must be some slight error" is an Understatement.
An Understatement is a literary device used by writers to intentionally reduce the importance or strength of a situation or event.
An Understatement, as a literary device, provides an ironic contrast to a saying or writing because the writer does not want to appear to be offensive.
Thus, the literary device that Gwendolen uses when she says, "I think there must be some slight error" is an Understatement.
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