If a shell hits the very top of a Pokey, his head is hit but he doesn't actually lose any segments of his body.
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https://www.youtube.com/watch?v=suzQZ5QuoEE
If Layer 2 hits the ground, Mario's spinjump, capespin, shooting fireballs, pausing, select and screen scroll sounds will be replaced by the "smash" sound Layer 2 makes.
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https://www.youtube.com/watch?v=o7y0-pR80cs
Found this completly by accident. I hit the walljump pixel and entered the pipe resulting in some glitched graphics.
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https://www.youtube.com/watch?v=aZviUSSacGo
You can overkill the Hawkmouth if you hit him enough times on the right frames. A weird glitched sprite keeps falling down that changes if toad picks up a block.
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https://www.youtube.com/watch?v=3oRezRlNiKw