When I filled out the ACA application last fall the income estimate asked for was a good faith (best guess) estimate. This is supposedly checked against prior tax year records and any major change should be explained (e.g. retirement, layoff, etc.). Exchange participant is also supposed to keep HHS updated on any significant changes in income or personal status (e.g. child birth, etc.) DURING the year.
Theoretically if someone were found to be negligently or (worse) deliberately under-reporting income (either on app or by not keeping HHS updated) they could be prosecuted under ACA. Specifically ACA states that anyone who "fails to provide correct information" through "negligence" or "disregard" is subject to fine up to $25,000. Worse, someone "knowingly" provides "false or fraudulent information" is subject to a "penalty of not more than $250,000". Those fines are "in addition to any other penalties prescribes by law". FWIW- ACA specifically says definitions of "negligence" and "disregard" to be used for determining violations are to be IRS Code definitions.
See pages 148-9 of ACA here:
And restated by IRS ACA-related eligibility info handling here:
Since IRS is agency responsible for processing/reconciling subsidies, I suppose this is likely to be handled like other tax matters. But since 2014 is the 1st full yr of subsidies, no one really knows how this will work in real life. Even if no new ACA-related legislation is enacted, I'm sure there will be new "implementation" rules/regulations issued by Feds (HHS, IRS, etc.) as time goes on.
IMHO- Best just to be honest on the application