out of curiosity, did ccexplore already know this?
I don't, apparently. But I tested quickly on Lemmix (editor), and it turns out other game-mechanics differences kind of fix the problem for Mayhem 15:
- When played using "Dos Original Lemmings" style, the mechanics (which emulates PC DOS Lemmings) spawns lemmings one pixel more to the left compare to other styles like DOS ONML or CustLemm. This is a well known difference/quirk with that mechanics, and in this case it is enough to make the fall safe for the right entrance.
- When played using "Custom Lemmings" style, although now it spawns at the usual x-position, that style is also known to have increased the safe-fall distance by 3 pixels, which is also enough to make the fall safe for the right entrance.
- The one Lemmix style where Ron's reported problem happens is ironically the "Sega Genesis/Megadrive" style I created specifically for the conversion, which uses alternate versions of VGAGRx and GROUNDx files to make the visuals look like the Genesis version. For that style I defined it in LemmixStyles.ini to use the ONML mechanics (presumably because I found it to be closer to Genesis behavior than Lemmings 1), which both spawns at the usual x-position and doesn't have the higher safe-fall distance, and testing today shows indeed the lemmings splat at the right entrance.
So by happy accidents, that level can apparently still work out okay in most of the configurations it can be played in back in the days. I guess that may be why no one reported a problem with this level all this time.