I was shocked to read this at Wikipedia's article on the Molotov-Ribbentrop Pact:
Lithuania, adjacent to East Prussia, would be in the German sphere of influence, although a second secret protocol agreed to in September 1939 reassigned the majority of Lithuania to the USSR.
Emphasis mine. The article has a cite for it too.
So the first version was agreed to 1939 Aug 24. This version had Lithuania assigned to Germany. Then the second version was agreed to in September, with Lithuania assigned to USSR.
Why the revision? Why so quickly? What prompted this change and why was it agreed to?
I'll also note that this was before the Winter War (Soviet invasion of Finland, begun 1939 Nov 30) that proved so disastrous, so it's not like they traded Finland for Lithuania.