Just make the law "any law with unintended consequences is illegal", then embrace any possible "consequence" of this law (such as perhaps laws you otherwise like being declared illegal), then the law should fail to outlaw itself. Any consequences would be by definition intended.

Anyway, I don't buy the suggestion that no law can be free of unintended consequences. If you construct a sufficiently formal definition of the system of law, and each law itself, then making a law without unintended consequences is simply (lol :P) a matter of proving the law. Quite similar to how programs can in fact be proven correct (contrary to the popularly held opinion that no program can be free of flaws.)

Is this practical? With our current setup, no. Hypothetically? Maybe... certainly at least worth pursuing the idea I would say.