The actress shared this cringeworthy story at a recent screening of the blockbuster, where there was a Q&A ... and where FP revealed there were some technical difficulties at one point in the production, and it came at the worst time ... during a sex scene.

If you've seen the movie ... you know there's at least 3 sex scenes where Florence and Cillian are nude -- so it's unclear which one exactly this happened for, but it sounds awkward.

Apparently, while they were filming this intimate scene, one of the main cameras broke ... and the whole crew had to hit pause while they worked on it to fix the problem. That left Florence and Cillian standing by (naked) as they waited for the issue to be resolved.

Luckily, FP had a good attitude about it all -- her whole point here is that 'Oppenheimer' was very much like an old-school Hollywood production ... and she's talking about how in awe she was by everyone and how they handled their craft. Even during this stop-down, she says she found stuff to appreciate ... and learned a thing or two about the equipment, etc.

Her point about the set being like a classic Hollywood movie isn't lost on us either -- considering Christopher Nolan's rep for wanting his cast/crew to be in the moment (and, allegedly, not to sit down either) ... this kinda makes sense that Florence and Cillian stuck around during this.

You'd figure the stars of a major film like this could head back to their trailers during this in-between time ... not so for a CN flick though, apparently! On his set, you see it through.

Anyway, sticking with it for these inconveniences clearly paid off for Florence and Co. -- 'Oppenheimer' is an Oscar darling this year, and is primed to win a bunch of awards.