So in Back to the Future Part II, Doc picks up Marty in 1985 and takes him to 2015. Then old Biff realizes the Delorean is a time machine. Old Biff steals the Delorean and travels from 2015 to 1955 to give the 1955 version of himself the 2015 sports almanac.
But when old Biff then returned to 2015 from 1955, shouldn't he have arrived in a way-different 2015? Instead, he arrives in the same 2015 that he departed, where Doc and Marty are still running around as if nothing changed.
update1: here's pic of Doc Brown in BTTF2 explaining how Old Biff created new timeline
update 2: After pondering more, I think this is a problem created by perspective. When Old Biff takes the Delorean back in time, we viewers do not go along for the ride. We're left hanging in a 2015 that becomes obsolete when Old Biff changes 1955. Seems our 2015 has two options: continue to exist as an alternate to the 2015 Old Biff created, or cease to exist. Either way, still seems our 2015 would be unreachable by Old Biff and the Delorean on their return from 1955.