Mechasaurus

Mechasaurus infamously has a "strange" hitbox. Many seeming direct hits will simply not do damage and dink off it.

The problem lies in the behavior of the Yamato spear. The spear travels 6 pixels per frame, but the boss's hit box is only 4 pixels wide, meaning it can be skipped.

As such, there are techniques to always land your shot. Specifically, walking off the platforms as they hit the ceiling, and firing two spears (to cover both angles) so one connects.