The first two entries in Sam Raimi's
Spider-Man trilogy focused on one iconic villain apiece: first the Green Goblin, then Doctor Octopus. For the third film, the filmmaker unwisely brought in three very different baddies the misunderstood Sandman, the vengeful New Goblin, and the shape-shifting bizarre-Spidey Venom. The result is an unfocused mess.
See Also: Batman & Robin,
Iron Man 2, and most egregiously,
X-Men 3. It's a surprisingly common problem for superhero sequels. Pray for
The Dark Knight Rises.