Oh goodie!!!
Another "have you ever"!!
GOOD question. Yes, I have ... in varying degrees. I'm probably more susceptible than some writers simply because I write stories out of order. So I have had to either throw out or heavily modify later sections because the story didn't turn out quite the way I expected.
Now I have to remember when it's happened!
<runs off to look at the fics>
This one is a little "off" from what you're describing, but in Heaven's Gate, I had an entire confrontation between John and Aeryn written, and threw it out because I didn't like where the build up to the conversation took the story. When I threw out both the build up to the conversation *and* the conversation itself, I gutted pretty much the entire story, including the title and the ending. I wound up sitting in front of the computer
thinking, "Okay ... now what?"
Originally, when John got frustrated and used the Rotarri Drive to cut their trip back to Moya short, they got
thoroughly lost. I had a scene where Aeryn's been sleeping while John tries to figure out where they are, and she comes back into the cockpit to find pretty much the entire floor covered with equations and hand drawn star charts, with John sitting in the middle. He figures out where they are, and they jump back to a spot close to Moya. Later in the story, I had a "reveal" where it comes out that John realized they were in Orion's Arm ... which is to say 'home'. Before he took them back to the UT's, he had already made the decision to stay with Aeryn.
I threw all of that out mostly because it was what I call a 'cheap' emotional trigger. It relies on everything built up during the show to create the pang, instead of doing the work within the story itself. I think you can see that by having John remain uncertain about what he wants to do right up to the end of the story, the storytelling works better in terms of tension and keeping the reader hooked. So there was a conversation all written that got heaved, along with a rather nice scene in the cockpit when John realizes what stars he is looking at (he's alone at that particular moment), and has to make a decision.
On the other hand, I never would have written that segment where he has to fix the engines, gets a bit frozen and they're in bed together ... which is one of my personal favorites.
I love John fixating on Aeryn's toes against the back of his ankle.
I've already cut a couple of scenes out of Measure of Devotion. It's not quite the same thing as not being able to get the story to a particular conversation, though. It's more like realizing that the conversation or the scene doesn't belong in the story, and making an often difficult decision to delete it. Now that I have the Wingnuts, I'm more aware of saving pieces that I decide don't fit into a story, and they will appear as "deleted scenes" at some point.
I think the real answer is that if a conversation is important enough to the story -- in terms of plot or emotional build up or whatever -- that a writer will keep plugging until they get the story to the appropriate point so they can use it. If the conversation, or ANY scene for that matter, is not critical to the storytelling or doesn't move the plot forward or takes the story in a poor direction, then it stands a chance of winding up on the cutting room floor.