Sorry, this is old, but I just was surprised by this and had to comment.... Or if they somehow lose their logo file because their computer crashed and they didn't have a backup, in the contract it would state that they have a lifetime guarantee from me. ...
Wow, you give a lifetime guarantee? Isn't that a bit dangerous? We actually make sure that the customer has a copy and tell them that we only keep the work for a year (unless it is an ongoing project like a website) and for them to make a backup, etc. We have so much work, that if we had to keep every job for eternity we would have to have terabytes and terabytes of backed up data which would be insanely expensive. That would mean if you happened to lose anything you would have to completely redo the job for free.
We actually normally keep artwork for longer than that, especially now that we can get a NAS server with many TBs for not all that much money. But still, if there were a fire here or whatever, to have to make sure that whole server was backed up off-site would be a nightmare. So we just keep the work until it starts to fill up, we start deleting the oldest work.