Time Turner
As much as I love Harry Potter, time turners have some serious flaws. They look very pretty and they're easy to transport, but time travel in the Potterverse is difficult. For starters, each spin is only an hour back in time. They can't be used for any heavy travel (anything more than a week back in time is extremely tedious). Additionally, it's difficult to transport more than two or three people at once because of the length of the chain. It can only go backwards in time (all the devices in the dropdown menu can go both forwards and backwards in time). These design drawbacks may be a result of the lack of time travel in the Potterverse and a general attitude to not change time. After all, "horrible things have happened to wizards who meddle with time," (Hermione Granger). Time turners aren't the worst design when it comes to time travel, but there's definitely plenty of room for improvement.