The Staircase Implementation - S3-E22
Corrected entry: When Sheldon tells Leonard about the time travel clause, he states "in precisely five seconds." Anybody who knows anything about time travel know that you need a definitive point in time, so without having that exact date and time written down somewhere, there's no way a future version of them could arrive at that unspecified time. Sheldon's too anal to ignore something like that.
Correction: The precise time would be slightly before the signing of the Roommate Agreement, which is about to happen, so it's a markable point in their histories/future histories.
Captain Defenestrator