In the Iran raid, Oper. Eagle Claw, 8 helicopters took off, one crashed, one malfunctioned, one helicopter turned back for still unexplained reasons.
This left only 5 choppers unable to ferry all commandos & hostages back.
With 6 they would have landed at embassy, raided, extracted, left and launched cruise missiles on Ayatollahs/leadersip ending their power, and supported a coup from remaining Shahs Officers.
All that didn't happen for that one helicopter; the Commanders on site are still bitter to this day and convinced the Islamic revolution would be a blip in some banana republic compared to what it became.
GQ has the story from 11/09; with all military involved.
http://www.gq.com/news-politics/big-issues/200911/iran-hostage-crisis-tehran-embassy-oral-history
Kyle: The first helicopter aborted due to a blade problem about an hour into the mission. Another pilot was separated from the group in the dust storm. He lost his confidence and went back to the carrier. He claimed he was afraid he was going to crash and all sorts of blubbering things.
Carney: They'd launched eight helicopters from the Nimitz. The one major contingency of the mission was that we had to have six. That was the absolute minimum. Six helicopters made it to Desert One.
Kyle: We refueled them. All were ready to go with Delta northbound.
Carney: Now you're high-fiving: "We did it—let's go!" And then it just turned to manure. One of the helicopters shut down; his backup hydraulic system was out. That left us with five helicopters—an automatic abort.
Kyle: The mission could not go with five helicopters, because the extra twenty-some people on the chopper that had aborted were too much weight. I was just trying to keep the mission going. I said, "Is there any way you can reduce by twenty shooters?" [Colonel Charlie] Beckwith said, "**** you, I ain't gonna do that. I don't know what I'm up against."
.....
Kyle: What it all boils down to is, one guy with a good helicopter—a helicopter we needed to complete the mission—turned around and flew all the way back to the Nimitz. The Marines nicknamed that pilot Turn Back.