30 years and we're still finding out things about the game that made the NES famous.
Despite its age, new secrets are still being found in Super Mario Bros. - including code errors.
According to a source code disassembly performed last year and published at The Cutting Room Floor, the behavior of Spiny eggs is not as intended. Rather than dropping down, the eggs were meant to be thrown at an angle based on Mario and Lakitu's position and movement speed, as well as having physics for bouncing off pipes or blocks.
The TCRF talk page indicates that a single, misplaced variable caused the error.
A side by side comparison can be found below:
