Revealing mistake: The can of "Chernobly" is supposed to be from Moscow, and the lettering on the can contains at least one Cyrillic letter. However, the name printed on the can is nonsensical, using random Latin and Cyrillic letters in an effort to look Russian, but the letters don't spell a word and means nothing in Russian.
Revealing mistake: While Jacob is playing Second Life in the beginning of the movie, shortly after the four-minute mark, the camera briefly cuts to the screen of his laptop and shows the controls for the video player they used to play back the captured clip. Additionally, in the same shot, the menu bar at the top of the screen implies a Macintosh computer, while Jacob is using an Alienware PC. (00:04:09)
Answer: It's just the rule they used for the movie; not a lot of thought was put into the time-traveling rules with the obvious inconsistencies the movie has throughout regarding it, especially in the ending.
lionhead