What should I do if a judge makes an incorrect ruling?
Typically you would appeal the judge's ruling to the head judge. The head judge is much less likely to make such a mistake.

In many cases, like at a local store, you won't have a head judge, which complicates the situation. This person's word will be final, so you will need to make sure they don't close discussion until you have persuaded them. You will want to talk to the judge away from the table (if for no other reason than to not disturb other players) and ask the judge which relevant rule he or she is citing to back up the ruling. I mean, unless you want to just drop it and accept that they flubbed it. I'm guessing it's implied that you would like to somehow contest their ruling.

Calmly explain why you feel the judge is incorrect, never lose your cool, and be confident. For example in this case you could say something to John the Judge, about your opponent Barry,


  John, Barry's Grasp of Darkness can only target creatures. If he wants to Grasp my Kiran, it must be a creature. For it to be a creature it must have been crewed. The ruling on Motorist says "Veteran Motorist’s last ability resolves before the crew ability that it was tapped to activate. The +1/+1 bonus will be in effect as the Vehicle becomes an artifact creature." With this in mind, the -4/-4 from Grasp would not be enough to kill it. If you disagree with this, could you tell me what rule or ruling contradicts this ruling?


What you don't want to say is "That's the dumbest thing I've ever heard." or "How long have you been a judge?" You're dealing with a judge, so think like you're in court. Anything that a lawyer on TV would say "Objection!" to, don't say it. Stick to the facts, stay calm, and if you still lose your case, just play on. Sometimes you lose when you shouldn't. If it's small enough to not have an L2 or L3 judge who knows these kinds of things, it's small enough to just have some fun and not worry too much about the prizing.