Where No One Has Gone Before - S1-E6
Corrected entry: While the starship Enterprise is (at first) flung only two million light years away, it is stated that it would take over three hundred years to return to the Federation. This is, of course, ridiculous, when compared with the time scale associated with the 70,000 light year jump in Voyager.
Correction: This wouldn't be a mistake on the Next Generation as much as it would be on Voyager, since this episode was made several years before Voyager.