"Are you sure you want to do that?" - You were until they asked. The tone and emphasis varies depending on whether its something that might be risky (even if the gm knows it will pay off) vs when it's something the character would obviously know not to do.

"You can try" - One of three possible responses to any question starting with "can I...", the other two being "that's not how that works" and "not if you want to continue playing with the rest of us."

"That is weird, isn't it?" - Something doesn’t add up. Is it a plot hole? Did the gm make a mistake? No, of course not, there's just more going on here than you know. Be sure to also use it when there really is something they don't know, as well as when they are just making wrong (but not game disrupting) assumptions. Say it often, knowingly, and with just a hint of smug satisfaction.