Sql-Server, Is null case-sensitive?

a co-worker of mine stated that she have a query that "work" only if she use NULL, and not with null.
She wasn't able to provide me with an example.

As far as I know there shouldn't be any difference, i even run a simple test:
select case when NULL is NULL then 1 else 0 end;
select case when NULL is null then 1 else 0 end;
select case when null is NULL then 1 else 0 end;
select case when null is null then 1 else 0 end;
and, as expected, they all returned 1.

So here the question, there is any know istance where using null is different than using NULL?