I spotted a sequence in an episode of Terminator: The Sarah Connor Chronicles which got me pondering on how it was made.
Cameron, the heroine Terminator, is sat drawing out a perfect barcode onto a security pass. It was so straight it actually freaked me out.

A whole lot of rotoscoping and garbaging later and I'd worked out how it could be done. Simple, but effective, the process involves starting with a printed barcode, then covering it in after effects and revealing it frame by frame.

On a few frames the pen didn't quite reach the end of the lines, so I repositioned the entire hand and pen in such a way as to suggest it did.

In an ideal world I'd redo this in HD with a non-reflective pen nib! The fact the pen was out of ink did help though.