'Fear Factor' Staffers -- Killing Donkey Semen Episode Was an Expensive Mistake

"Fear Factor" staffers aren't just pissed NBC execs pulled the plug on their donkey semen episode because it's hilarious -- they're pissed because ANOTHER stunt on the show was extremely expensive to stage ... and now it's going to waste.

Sources connected with the show tell TMZ, in addition to the donkey semen stunt -- contestants had to jump a car through a moving train ... a stunt that cost roughly $150,000 to put together ... and now, will never see the light of day.

On top of the train disappointment -- we're told some staffers were baffled by NBC's decision to yank the episode ... because the semen challenge was "hands down the nastiest concept" out of the bunch of other stunts that were killed BEFORE coming to fruition.