Bava Batra 162 - The Last Line in Any Legal Document is Ignored
When witnesses sign on a document, they leave approximately one line of space between the text and their signatures, to insure that their signatures do not run into the text. There is therefore a concern that an unscrupulous individual might insert a bogus line of text into this space. To protect against this possibility, the Sages decreed that nothing is to be learned from the last line of a document, and any new information there is to be ignored. Therefore, the scribe uses the last line to superfluously restate the document's essential elements.
If the witnesses leave a space of two lines, the document becomes invalid. However, if they fill the space with the signatures of witnesses who are either relatives or otherwise ineligible to be witness, such as gamblers, the document is valid, similar to the fact that a hole of three hand-breadths in the covering of a sukkah invalidates it, but a patch of covering made of unfit material can be up to four amot.
Art: Edwart Collier - A Trompe L'Oeil With A Pewter Ink Stand, Books And Papers