In T-SQL there is no guarantee about the order of how OR and AND statements are evaluated. Of course precedence rules are observed, and the result will also stay correct, but the optimizer may change the order arbitrarily when it is allowed.
it is true that evaluation order within an expression cannot be predicted, but you can pretty much depend on the scalar comparison to a literal value (@s = 1) being performed first. take the longest running query you have and add "and 1=2" to its where clause and it will fly.