When a user submit a form that contains a FileUploadField, I decided to disable the entire form so that they couldn't change any values while the upload was happening.
This caused a horrible side effect -- it made the form not submit any values but the FileUploadField itself..
None of my Hidden or TextField values were being sent .. this took me (no joke) 8 hours to solve .. I drank a lot of coffee b/c of this
edit: I resorted to masking the form instead of disabling it -- not quite as pretty, but it'll do.