This was just an exploit of crappy programming and an ineffective paper log on Sequoia's part and not an indication that the principle of a paper trail is flawed. For instance, in the case when the voter leaves before actually casting their vote and it is then voided could easily be avoided by making the voter aware that their vote hasn't been cast until X. If they don't bother to ensure that X happens, it's their fault. It's equivalent to handing your ballot to someone without staying to watch them drop it in the machine. If that is still too complicated for people, there can still be an optional paper equivalent.