What do you do if you find out that your health insurance agent falsified your information?

Unless you are on medicaid or receive a subsidy for your health insurance through the marketplace...there isnt any concern here.  

If you do, then you should be prepared to pay back $300 per month for that timeframe.  Honestly though, they probably just caught the issue at renewal and let it go.  

Back pay? When you bought insurance? You are confused.

You signed it, so it's on you.

If the form has your signature on it, you're responsible.   As an alleged grownup, you need to READ a form before you sign it, and also make sure that there are no blanks.    

Yes, if you got an Obama care plan, the cost is re-evaluated at the time you pay your taxes, to adjust for your actual income. 

If you were on a income-dependent insurance, it sounds like your insurance was purchased through the federal or state marketplace. You would have received a 1095A form from your insurance company that had to be reported on your tax return to reconcile the amount of subsidy you received vs. the amount you were entitled too. If you received too little subsidy, you got a tax credit. If you claimed too much, you owed the difference which would have been subtracted from any federal tax refund you might have gotten. If this was not included on your tax return, you still have time to amend and correct the situation. I would file a complaint with the agent's brokerage.

They can & will come after you at tax time IF you significantly underpaid last year.

Come after is not really a good term. The tax software will add it to what you owe.

They only come after you if you don't pay.

The first few years, I overestimated and got refunds. Since then, I never overestimate and pay zero.

But if I have a lot of stock gains one year, I could end up owing.

You are responsible for the content of your insurance application and/or subsidy application.    If you didn't bother to review the application, that's on you.

Yes, if you received a subsidy that you weren't entitled to, they may issue an over payment which you will have to repay.

are you sure?

My guess is that you were on the high end of eligibility to receive a subsidy and this year you no longer qualified.  In my state, that makes it about a $300 jump per month on average (based on age and other things).

If your income was wrong and you received a subsidy when you shouldn't have, you will be required to pay back any money you received when you file your taxes. 

Just like with an accountant, it's your responsibility to validate any information that is provided to an insurance agent.