[wladimir]

Even better, get rid of your forms entirely, especially long forms with all kind of questions that you really don't need the answer to. Or have fields that are ambigious and improperly validated. Or multipage forms that have no way of going back without losing your filled in data.

[narag]

Specially annoying is when the "check this if you don't want to receive spam from us" checkbox is unchecked every time the form is shown again after a server-side validation error.

[wladimir]

Oh yes, I agree. Or when you mistype the essentially human-unreadable captcha, and have to fill in half of the fields anew.