Deliberate mistake: When Old Biff and Young Biff are sitting in Biff's car, the car's rear-view mirror vanishes during all shots of Old and Young Biffs together. Presumably this made the split-screen effect easier to do.
Deliberate mistake: When Eric's ship blows up, his statue bobs up and down. A statue this weight would not be able to bob up and down for even a second. It would just sink straight to the bottom.