For several years in grad school I had income that varied significantly in different parts of the year (I got a large scholarship check as a lump sum). So every

For several years in grad school I had income that varied significantly in different parts of the year (I got a large scholarship check as a lump sum). So every year, I had to fill out some obnoxious, moderately complicated form detailing my month-by-month income and calculating the estimated tax that I needed to pay each quarter. Upshot: the tax system does allow for changing circumstances and won't penalize you for not foreseeing them in advance, but it does make your taxes a lot more complicated.