So, what's the big mistake?
What if you adjusted the goal to, "Over the next three months, I'm going to implement a sustainable system of tracking my nutrition and exercise, with personal accountability built in." It's still a SMART goal, but this one serves you for life and still leads you to lose those 30 lbs and probably more. The only difference is that this one changes your habits so you keep the weight off and stay healthy.
One more example. "I want to get promoted to general manager at work by December" could be re-worked into "I will work with a mentor or coach to learn and implement new personal growth habits and new habits at work by December." Version one might get you the promotion, but then what? Version two transforms you into a person who's promotable for the rest of your life.
Of course, this assumes you care enough to take the time and make the effort to set goals in the first place. If that's a problem for you, you can read this article I wrote a while back. It may be a good start for you. Hey, I hope you've got goals and I hope you're making daily progress building the habits that will get you there. You can do it. Nothing, but you, is standing in your way. Go!