Under the Affordable Care Act, people who meet certain income eligibility criteria will be eligible for subsidies to offset costs of premiums and cost sharing for health insurance plans purchased through new health insurance exchanges. But determining the correct level of these subsidies will not be easy, because of several factors. These include the way in which eligibility will be calculated for participation in Medicaid or for subsidies through the exchanges; possibly inaccurate income projections; the use of different income time periods to determine eligibility; and fluctuations in income. I performed a simulation that shows that under the most likely methods to be used to determine eligibility for Medicaid or for receiving subsidies through exchanges, one-third of people with incomes initially judged to be below the Medicaid threshold would actually "churn" into an exchange at the end of the year. Other people would be wrongly deemed ineligible for advance subsidy payments because their projected income was too high, while still others judged eligible for subsidies would receive advance payments on those subsidies that were too high by $208 per year, on average. To reduce these errors, I recommend the adoption of a single eligibility standard based on income data derived from prior tax returns, along with generous accommodations during a given enrollment year for people who claim a change in circumstances, such as a change in income.