This issue depends on which quarter of the you receive the unexpected income. If that unexpected income occurs in a later quarter of the year, you might not have made large enough quarterly payments in earlier quarters to accommodate the unexpected income without in underpayment penalty unless you make a payment for the quarter that the income was received that is large enough when combined with the earlier payments to accommodate all of the tax liability incurred to that point in the year (because you will need to annualize income to avoid the penalty). However, if the unexpected income occurs in the first quarter this income won't be unexpected at the date of your first estimated tax payment when you can calculate the amount of equal quarterly payments needed to sum to your overall tax liability for the year (avoiding annualizing income).
