 PLOT: When Marty and Doc were discussing the alternate 1985 Doc  shows Marty a newspaper headline that reads "Emmit Brown Committed"  that appears to be from May 1983. If Doc was committed he couldnt  have built the Delorian, therefore none of the events could have  happened. If none of these events could have happened then Old Biff  couldnt have taken the sports almanac back to Young Biff resulting  in Doc getting committed. A time paradox all in itself.