Here's a quickie: We let people pick up copies of Tranhuman at Gen Con. In our backer surveys, we asked if they wanted to do so, and from that data we created a handy checklist of people so they could sign off on their copies and I could import that data back by hand into the Kickstarter database later.
Of course, a few people showed up at the booth and wanted to pick up their copies, even though they didn't say they would in the backer survey.
That was no real problem; we verified that they were backers, had them sign off, and I added that information to the database later.
Here's the hiccup: one of the people that signed off on their copy has a relatively common first and last name, and there are two people with that exact same name registered as backers. Which one of them picked up their copy? No way to solve this except to go right to them and ask—hey, did you?
In the meantime, I realized that during the first wave of shipping one of those people had a copy of Transhuman shipped to them; and as luck would have it, that person was the one who picked up their copy at Gen Con. So they'll get an extra book for their gaming group, we eat a few dollars in shipping costs, and life goes on.