1) I didn't imagine this scene
2) This scene was only shown in a few preview theatres before the major release... so it is understandable if its not considered part of the original widespread release
3) No one has been able to post an actual copy of the scene or photos of it which means.. Even Lucas may not have it anymore.
Here's what I found
End of recent edit
From the site:
....An entirely different group of people seem to recall seeing Luke at the chasm throw his grappling hook and miss the first time, then throw it again before swinging across with Leia.
And from the site:
One persistent rumor is that the Star Wars CED contains a scene where Luke misses with the grappling hook the first time he throws it across the chasm in the Death Star and has to throw it a second time. I've checked about a dozen Star Wars CED's for this scene and haven't seen it, although you may want to check your copy (the grappling hook scene is 25 minutes into Side 2). This missing footage may have been present only in the 70mm theatrical print shown in nine theaters during the original May 25, 1977 release. More information on this is available in the Star Wars Compendium of Lost Footage.
Edit: and also from the site:
Subtle changes to the film since the initial 70mm New York (Loew's Astor Plaza) and Los Angeles (Gramann's Chinese Theater) showings... (I must point out that these are from memory, and I have only been able to verify them by asking the people whom I attending my first showing of the film with, and they have concurred...)
1) When Luke threw the grappling hook, he missed the first time and had to try again.