Unit tests and parameter validation

Hello everyone -

I'm curious to know thoughts on writing tests that specify the behavior of
a method when its inputs are incorrect.  Is this a common practice.  How
about parameter validation in this case?  Having some healthy debate on it
and wanted to get other insight.

The context for this in a class/methods that are used within an internal
application - this is not an API that will be publicly exposed to external
consumers.

Thanks.

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