Update from May 4th 2018 : ESA has started planning for a global reprocessing in 2019
The information gathered by Sentinel-2 system on Sentinel-2 orbit, attitude, date accuracy, and viewing directions of all detectors allows an excellent accuracy for the geolocation of all Sentinel-2 pixels. The overall geo-location accuracy is better than 11 or 12 meter, for about 97 % of the cases, which is about the size of one Sentinel-2 pixel. Such a performance is more or less equivalent to that obtained for Pleiades, but Pleiades has a resolution of 0.7m. It is therefore really an achievement, which is to be credited to ESA, to the satellite and instrument manufacturer, and to the image quality teams (including my CNES colleagues).
But even if it is excellent, it is not enough. The standard need for multi-temporal registration errors is 0.3 pixels, and the current performances show that for more than 50% of the cases, the performance does not meet that requirement.
For many users, I guess, these figures do not mean much, and it is not easy to figure out their impact on real life applications. The animation below (made by the twitter star Simon Gascoin) makes that much more concrete :