If you earn just one penny over the maximum income permitted in order to qualify for a new tax credit on the purchase of health insurance, you could lose the entire tax credit. And the tax credit does not phase out, it falls off a cliff.
For example, for two adults both age 50, they could qualify for an estimated annual tax credit of $5,136 so long as they make no more than $62,039.99. At $62,040, they lose the entire tax credit. The calculator is online at: http://www.coveredca.com.