So I watched (a bunch) of that video and the evil invisible blocks that kept killing them aren't supposed to be there. (Or at least not most of them.) So now I'm wondering who screwed up? Does one of the ASM hacks (some of which add extra blocks) have a bug? Is the level data bad? Was the bug introduced by the X1 hack? (Is it an (unlikely) emu bug?)
I guess I'll have to go investigate.
So I can confirm that the worst offender (the one on 1-3 that killed Poe and Mato repeatedly) isn't in the version I have sitting around (which makes me doubt any of the other ones are as well). Just to be sure, I downloaded the released patch and confirmed it produces the same ROM as the one I tested on. I looked into the X1 hack and verified that the raw level data is completely unchanged (which is not surprising given how involved editing the level data became with all our changes).
My current working theory is that there is a bug in the X1 hack that somehow borks the object drawing routines, causing the spurious blocks to appear. The level loading stuff was rather extensively modified, so hacks that work on vanilla SMB wouldn't necessarily apply cleanly to SMBS. I still have to actually test the X1 version and confirm the behavior. However, I have to go to work, so I'll look into it later tonight.
So, I tried to reproduce the bug in the X1 (style) version, and that invisible block in 1-3 doesn't appear. I don't know what was going on. Oh well. If anyone else has any ideas, let me know.