I think I figured out what was happening. It looks like the tightening process somehow cocked the rocker so that it bound against the cam. I was able to wiggle the nut with the socket and got it to free. Now the gap appears to be set correctly. I spun the crank two full turns to recheck and its still good.
Here is a link to a guy who says he broke his while tightening the locknut.
I am trying to figure out how tightening the locknut can stress the rocker so much that it cracks. Based on that guy's photos of his rocker with broken dog ears, I am thinking that too much torque can torque the entire rocker, not just the nut, causing the dog ears to snap off.
I checked the dog ears on the problem rocker and they are both still there. I'm glad it didn't break. Replacement ones are $50 each plus its a big job pulling the cam out to replace it.