To me it's always sounded like a rough demo, certainly not fully rehearsed. You can hear the musicians feeling their way around the song gingerly rather than playing definitely rehearsed lines and notes. Maybe 'L', Leonard, Larry or whoever turned up with a sketch for a song and they just jammed it for a couple of minutes with the tape running and as it didn't sound too shabby, left it at that.
The bit in the middle where it goes funny sounds to me like the drummer and bass player getting a bit lost and not knowing when to come back in for the singing. I've done that plenty of times in my dark music playing past!!!