For example, there's a scene in the Millennium Falcon where you can see Harrison Ford mouth-prompt the next line to Fisher with the story being that director Irvin Kershner threatened to fire her from the set if she flubbed the line one more time. Another story was that you could visibly see Fisher's cocaine nail in one scene during 'Return Of The Jedi', though she herself denied this was the case - but didn't deny she was high on cocaine for a lot of the shoot.
Another story that Fisher told was how during the sequence set on Cloud City with Billy Dee Williams, she and Harrison Ford were still reeling from the effects of something called the 'Tunisian Death Drink' that was given to them by Eric Idle and the Rolling Stones. Keep in mind, this is '70s Rolling Stones, right in the middle of 'Some Girls' and all their deep, deep drug use.
Anyway, Fisher told the story to Daily Beast back in 2015 that she and Ford, after a night on this particular drink that was most likely laced with some drugs, rocked up to the set and were still buzzing on the stuff and the takes that made it into the movie shows Fisher and Ford trying desperately to keep it together.
Here's the scene in question.
In a recent interview with The New York Times, Idle backed up Fisher's account of what happened and said that, yes, Ford and Fisher were both pretty much wasted during the whole scene on Cloud City. "We had a party the night before. We stayed up too late. We were having too much fun. Carrie Fisher had rented my house and she was staying there. We went to bed and they went to work. It turns out when they filmed the scene, they were still a little high. So yes, I’m proud of that moment," Idle explained.
To be fair, when you look at the scene, you'd have no way of knowing anybody's on anything, so it's really just a showcase for how professional Harrison Ford and Carrie Fisher were that they could manage to string a few words together and walk down a corridor and not fall over.
Pros, the pair of them.