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An illegal argument exception in MongoDB aggregation occurs when the operation expects to retrieve a single unique result or null, but it ends up returning multiple values. This usually happens when the aggregation pipeline or the parameters passed to the operation are not properly configured, resulting in ambiguous or incompatible conditions. Some common causes of this error include trying to use an aggregation operator that requires a unique input but receives multiple values, using a group by clause with incorrect or incomplete parameters, or specifying conflicting criteria for filtering or sorting the results. To resolve this issue, you should review your aggregation pipeline and the query parameters to ensure they are correctly configured and aligned with the expected output.