Im pretty sure that WAS lava in Junkman's stage.
Well, um, aside from:
[...]I'm not counting the stuff that stops you dead in Junk Man's level because that's slag, rather than lava[...]
[...]There were shallow pools of slag in Junk Man's level[...]
And interestingly enough:
Im not talking about any junk man thing[...]
Every time I look for lava, I find obvious mid-to-dark red in conjunction with the bright orange, and often some black. When I look for Slag, I only find bright orange to white. That makes sense when you consider that Junk Man's level appears to be junk metal management plant; I can't imagine why he'd have LAVA there. Also, it takes at least twice the heat to liquify rock as it does some metals, and the surrounding area itself appears to be made of some sort of metal, which doesn't seem to be affected by the heat at all. That could be an argument against making the slag instant-kill in the first place, but one could see how it could be less deadly for Megaman in any capacity. SNES-style comparison:
You can't see the floor under the slag, but I get the impression it's not that deep anyway, especially considering that the red bit immediately below the floor you stand on appears to be one of the pipes it's flowing through. Also, it has to react to Freeze Cracker bullets, so it may only be solid for the sake of saving cycles while testing for collision with the initial, large bullet, which is already testing for collision with solid tiles. Performing one solidity test during movement, and testing for the special slag tile IDs only once (at the point of collision) would save a little processing time. Also, as far as its reaction to Megaman goes, using the already-existing instant kill property for spike tiles would save any time it would take to make a special variation for the slag, and would also prevent Megaman himself from having to perform two tests instead of one for every frame that he's in contact with a solid surface, again, saving a little processing time. The slag was actually used so little that that aspect of its functionality probably just plain didn't matter so much. Also:
As little sense as it makes to have an actual bottomless pit in an inside area like that, the level did also have graphicless instant-kill pits. What we chose to do was fill all of our "bottomless pit" traps with lava graphics, because the level does take place in and on a volcano, after all. It's not even about "well I want some pretty lava in here so I'm just going to put in the graphics regardless of how it actually works and then see what I'm supposed to do", this is the mechanic that we wanted when the level was actually being set up. The effect is that the pools are so deep that there is no way out. Megaman is just going to get stuck in there and sit until he absorbs so much heat that even if doesn't actually melt completely he's still just going to blow out and die that way. That's not to mention the fact that in every original series game (pre-MM9, anyway) that has a large pool of actual lava, you cannot stand on it, and nothing will protect you from death
And.....wait, what?
If it was instant kill, it was lava.
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Oh, sorry... I apparently have absolutely no idea what lava is. I didn't realize it was so prevalent :X