Nice job getting the shard! It's possible to get out but it requires a very precise jump and downburst. Also, you can also get to it without taking damage with precise downburst movement, but that's not the intended way to get it (you will need to wait for the full game to figure it out!).
That's a nice first Metroid game, wish I could forget all about those and play them again!
My initial idea was to make the spikes reset your character to the initial position (think like Hollow Knight does it, if you've ever played it), I'm still not quite sure if that's better than just having them damage the player... will need to playtest more to decide on that!
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• That's an interesting idea, but I think it would lead to waaay too many pages, because GBStudio cannot handle too many sprites in a row without visual glitches, but it's true that it would look way more appealing that way...
• Will need to investigate the Karkanon glitch, thanks!
• Do you remember by any chance which screen the weird HP drop happened in? It's probably a wrong script on a specific screen, I'll look for it!