suppermariobroth:

In the Forest Fortress in Super Mario World, it is possible to defeat a Grinder by sliding into it using the Triangular Block. Since Grinders are not supposed to be defeated, its sprite becomes glitched as it falls offscreen. (Footage recorded by me from a SNES emulator.)

suppermariobroth:

In Super Mario World, taking a Buzzy Beetle through the exit pipe in Donut Plains 2 results in the Buzzy Beetle’s shell assuming a grey color scheme. In addition, no matter how much time passes,
the Buzzy Beetle will never emerge from its shell again. (Footage recorded by me from a SNES emulator.)

suppermariobroth:

In Super Mario Bros. 3, the rock between the path to the first fortress in World 4 and the Spade Panel can be destroyed with a Hammer item, allowing the player to bypass the fortress. There is no obvious indication that this rock is destructible, as it is surrounded by decorative rocks. In fact, many guides for Super Mario Bros. 3 do not mention this shortcut. (Footage recorded by me in a NES emulator.)