Just found out that after clearing Bilewater I apparently permanently missed out on a
Zero Mission-style segment in The Slab because I killed the jailer that would have taken me there
. That's a bit of a bummer, would have really liked to see that!
You can get it still. It spawns in multiple locations, but randomly. I'm not sure anyone knows quite how it works.
I think if you get there naturally it may be too late though? Unclear on that.
"One toot on this whistle will take you to a far away land."
-Toad, SMB3