Great question and great concept.

I added COL1 to TblA2 and expected "Ambiguous column name 'col1'.". But this time Sql server selected column from TblA2 and gave me the correct result.

It will be failed in Joins if we didn't prefix the table with alias name.

But as Hugo said, we can't predict the output if someone add col1 after sometime. So always it is better to prefix the columns with table alias name.