The allowable Premium Tax Credit for your Obamacare insurance is calculated based upon a percentage of your actual income for the entire year.
If, for instance, you only had Obamacare for 6 months, and you weren't far off in estimating your income, but then you got insurance through your new job, you could end up owing everything back.
This is particularly painful for people who were out of work when they got their insurance, and then got a new job. It's one of the flaws of the law as currently written.
