1st, after Curly falls through the bathroom floor and lands on Moe, later when Curly goes back upstairs to the bathroom the "cage" of pipes he created is gone. 2nd, when Curly first goes into the bathroom to fix the leak, he knocks on the shower curtain, you can tell by the sound that he's banging on something solid. What solid surface is there behind the shower curtain?
shareThe sound was a Foley effect. His cage of pipes was probably taken apart by Moe or Larry.
As far as the missing pipes, maybe they came down with Curly when he crashed through the floor...anyway, the pipes had to be gone given the rug over the gaping hole in the floor gag that followed.
As far as the solid surface behind the shower curtain, that was part of the gag. Curly arrives in the bathroom to find someone taking a bath behind a shower curtain. He knocks on the curtain, which seems silly, but to the viewers surprise there's a solid surface that creates an actual "knocking' sound, hence the gag, not a goof.