I kept receiving this error when I tried to authenticate my iPhone app:
Woah there! This page is no longer valid. It looks like someone already used the token information you provided. Please return to the site that sent you to this page and try again ... it was probably an honest mistake.
Google searches for a solution only pointed to other people with the same issue. I had no idea what was happening as it had been working 30 minutes prior.
The problem turned out to be related to the sytem time on my device. Due to some previous testing, my time was off by a day. Readjusting the time fixed the issue.
You may encounter the problem while using any service or client that relies on the Twitter API. Check your system time and resync with the official US time and see if it helps.