Besides the FBAR (the Report of Foreign Bank and Financial Accounts, Form TD F 90-22.1, warning it is a PDF) the single most “oops I forgot” form that I see for regular human being clients is Form 3520. Gifts received from nonresidents aren’t reported properly. Or at all.
Something tells me (warning! PDF) that the IRS is going to turn their radar to this area next. The IRS is soliciting comments to Notice 97-34 (PDF). That can only mean *ahem* improvements are coming in this arena.
The current FBAR amnesty program will clean up the failure to file Form 3520, if that’s something that applies to you. Highly recommended.