After I finished level 4 in Route 66, I wound up in a level from Redneck Rampage 1. WTF?? Anyone else encounter htis?

May seem a silly question, but were you running the RR executable? Route 66 is an addon pack and the saves seem to appear in the original verison as well (probably a lot of shared files between the two). So, if you loaded your game while running RR (and not R66), when finishing the level you were on you might have ended up back on the RR1 level track.
Just a thought.

No, after that level it was all R66 levels again. The level was called "Flea Market" or something like that, but there was nothing Flea Marketable about it. Some kind of strange bug.

Interesting, which RR1 Map does it take you to? Can you perhaps get a screenshot or the original RR map name?

It was Dairyair farms. The one with all the enemies right next to you when you start out....
I think I know what may have happened. RR1 and R66 can load each other's saved games. So maybe I accidentally started up RR1, continued with my R66 saved game on E1L4, but on finishing the level it loaded E1L5 from RR1 instead of R66.
Ah well, now to make my victory over R66 complete, I'll just warp to the real Flea Market and beat it :)