Lookups - multiple hits

An underlying assumption of the lookup task seems to be that there will be a unique record matching the lookup criteria. However what happens when more than one record is matched? Does it return the designated field of the first record encountered, or does it error out? Is there a way to specify what SHOULD happen in such a case? Is there a way to specify secondary criteria if the primary returns multiple hits?

Related to this - suppose that my lookup returns 2 data points, but one of them is null. It seems that the lookup should appear successful, and rows can then be redirected based on which field was null via the Truncation setting.
For example, say I'm matching on "name" and returning "Id" and "description". The name is found, and there's a valid Id, but the description is null.
Would this result in a successful lookup with the "description" truncated, or would the lookup fail?

Thanks!
PhilIn full cache mode the lookup will report a warning and use one of the records. In partial or no cache it will use one of the records with no warning. I say one of the records because this is an implementation detail and is subject to change.

You can always specify more than one join column so that you will not get multiple hits. Obviously, this is not exactly what you are asking for because it sounds like you only want the secondary match to occur if the primary is a dupe. This the lookup does not do.

NULL does not mean truncation. In the scenario you specify the lookup would be considered successful with a description that happens to be NULL. Trunctation is when you try to put a string that contains 100 characters into a column that is less than 100 characters.

Thanks,
Matt