I was fortunate to see the workprint for ST IV, before the final version premiered in Australian cinemas. Hilarious unfinished FX work!
A lot of people forget that the original prints for ST II, used in the first few weeks of US release, then sent to Australia and other locales for their
premiere weeks, deliberately had no "II" on the title card.
The dropped scene with Katherine Blum's little Vulcan girl, telling Spock to "Live long and prosper" in ST III, is a weird one. We saw the film in several sneak previews in Sydney, plus on premiere night. We got to know it pretty well. The girl's name is listed in the credits, but the scene wasn't there. But... a friend was in Perth for opening night. When she got back to Sydney, we were discussing the film and she swore
she'd seen that scene. I've never been able to get anyone to corroborate her story.
A brief scene of the Federation President's consort/female aide is in the ST VI trailer, but not in the release print.
As for people misremembering what was in "Generations"... Wasn't some
of the (eventually dropped) footage previewed in "Entertainment This Week", the weekly international compile of the US show, "Entertainment Tonight", long before premiere night? I know there was Kirk, Scotty and Chekov stuff that was aired on that show, but not featured in the final movie.
A lot of people don't realise that there were two versions of Kirk's demise in the novelisation. The original hardcover had the scene that received the bad preview reactions (and is in the bonus scenes on the DVD). The paperback version, published many months later, has the final, reshot version shown in the theatrical prints.