There has been an incident where Fox has dropped entirely a ready-to-air produced episode. It was a 1989 Married with Children episode. Here is a snippet from the MWC faq http://world.std.com/~tob/mwc.faq
----------------------------------------------- Subject: 3.3. What was The Lost Show?
"The Lost Show" was slated to be episode 310, "I'll See You In Court". The producers could not come to an agreement with the network censors, and the episode has never aired in the US. It has aired in Europe.
Producer Michael G. Moye has said about the episode: "If it aired today people would wonder what the problem was." -----------------------------------------------
I remember an interview with David X. Cohen describing the Amazon episode as rather raunchy which may make it inappropiate for the 7pm slot that some consider family viewing. My feeling is that Fox sent back to the cutting room so that it falls into moral guidelines that have not changed since expectant tv moms were allowed to use the word "pregnant".
The conclusions is the 7pm slot sucks big time. It's a lousy slot to begin with, it gets constantly pre-empted by NBA amd NFL, and it's way too early for an adult-themed show.