The column name in the temp table is missing an I, probably just a typo, but it has some rather pronounced effects.
The obvious next question is why the select with the subquery in it didn’t fail, after all, the query asks for ClientID from #TempClients, and there’s no such column. However there is a ClientID column available in that query, and it’s in the Orders table. And that’s a valid column for the subquery, because column binding order, when we have subqueries, is first to tables within the subquery, and then, if no match is found, to tables in the outer query.
I know that the first time I experienced this, I thought it was a bug as well. But no, it’s by design and Gail explains why.