The other day my iPod froze after a workout. I'm not sure why, it's been mostly on a treadmill, not on my waist during runs, so it shouldn't be getting jarred too much, too wet, etc. In any case, it froze, and wouldn't sync with the computer.
So I reset it, tried again, and my run was gone from the day. Not a huge deal since I'd paid attention to my stats, and so I could enter them in MapMyRun.com. But it was annoying that I couldn't add this to my Nike+ account.
There's a fundamental issue with Nike +. Technical issues, loss of power, any number of things might cause your run to not be recorded and you might still want to capture that data. Right now, Nike + ends up showing incomplete data.
And for anyone that wants to track their runs, the idea behind Nike +, this is incredibly annoying. When you start to lose data, you stop wanting to store data in the system.