I remember the first time I had an effect go south on me on set. It was my very first slit throat on my very first feature, and I had watered down the blood mix too much. So embarrassing! In the world of ultra low budget film, there is rarely money or time for tests. Sometimes you will be asked to pull something out of thin air right before a shot.
One of the funniest but scary moments I had was a director asking if I could do an eviceration scene on a guy that was going to shoot I n about 20 minutes.....because they changed the script but forgot to tell me. We ended up making intestines out of pantyhose, cotton, and gelatin on a tarp....took a female autopsy prosthetic and flipped it upside down with the breast tissue removed....and made it work.
You never want to be put in that position.....but some days you will. So when it comes, just roll with it and try to enjoy the rollercoaster.