Let's see....
Forgot to load film.
Forgot the meter.
Forgot the cable release.
Forgot the dark cloth.
Forgot my brain.
Mismarked holders
Mismarked my brain
In cold weather, couldn't get the darkslide back into the holder.
After a 5 hour night exposure flipped the flashlight on and ruined the film, before replacing the darkslide(last weekend).........was very sleepy.
Only ruined one box of film.....of course it was a box of 8x10 Velvia.(expensive stuff).
With more and more practice, mistakes (should) become fewer and farther between. This year I have only ruined two shots, the long one last weekend and another due to the film not have been properly loaded into the holder.
Keep shooting and keep practicing, especially clean your holders before loading them. Most of your dust problems will vanish quickly.
Good Luck.
Gary