Many great missed opportunities as have been mentioned here. I can't count the number of movies I watched hoping for something to happen and it never did. My list is a lengthy one but we'll moderate it here in the interest of plausibility.
Swamp Thing - Adrienne Barbeau made Heather Locklear look like a little girl. The scene should have come right after she was bathing and walked nude into a bog that took her under before "Thing" pulls her out.
Maureen O'Sullivan in any Tarzan flick. According to legend and some fragmentary documentation such as the photo below, a terrific scene was shot for
Tarzan Escapes but was deemed too intense and left on the cutting room floor. Jane is a mess as you can see in the photo.
TarzanEscapes.jpg
Anaconda - When Carrie Whurher went off the boat into the woods alone I was positive that was the moment. It would have been better than John Voight breaking her neck with a scissor hold.
Ava Gardner in
The Naked Jungle. One of Hollywood's all time beauties misses out.
Raquel Welch in
One Million Years B.C. Epic, epic, tragedy. Could have changed the world as we know it.
Any one of several intentionally so-bad-they're-good soft core porn flicks by Fred Olen Ray. Quicksand is even mentioned as a potential hazard in
Bikini Cops Go Hawaiian. Then the leggiest porn babe goes off by herself only to find a perverted old jungle guy.
What if Princess Leia had gone to Dagoba?
There is one huge obvious scene that should have happened at least once. Mary Ann and Ginger on
Gilligan's Island. I can't count the number of times my adolescent mind played with that one.
I could go on but those spring to mind right away.
EDIT - DOH! One of my all time wants was Gena Davis in Cutthroat Island. Instead her dumbass boyfriend ends up in the quicksand. The idea of missing Gena Davis in pirate garb going down is a tough thing to swallow.
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It's a dirty job but I got to do it for over 20 years. Thank you.