Star Wars: The Last Jedi had several big moments, but none was likely as big as the moment the audience discovered that Luke Skywalker's battle with Kylo Ren took place without Luke actually being there. Luke was actually using the Force to create a vision of himself which appeared to be the real thing. However, there were some clues to the reality of the situation which, as Rian Johnson reveals in a special feature on the new Blu-Ray for The Last Jedi, he placed on the screen for anybody who might have had the presence of mind to notice. According to the director...
Well, there's little things that we put in there, like the salt flakes that are falling don't stick to him at all. They don't interact with his lightsaber. There's actually a very specific shot where we cut to his feet, and as his feet are moving along, we had ILM repaint the ground so that it doesn't affect the salt on the ground.
Most people probably didn't notice any of this because the focus in that moment for the audience is on the encounter between Kylo Ren and Luke Skywalker. You're so focused on what's going to happen you don't notice the details, which is, of course, the point. However, when watching Star Wars: The Last Jedi again, something which is now easy since the film is available in both Digital HD and Blu-ray formats, you can now go back and see what you missed.
Of course, you can look at these moments as hints of the reality of the situation, but they're also necessary for film continuity. If Luke actually had bits of floating salt sticking to his robe, fans would be nitpicking that after the fact once it was discovered that they shouldn't be, since he's just a projection and not the real thing. Many fans were already frustrated by the entire concept of Force Projection, due to it being an ability never seen on screen before. Certainly, if there were continuity errors in addition, they would have been really upset.
I certainly didn't notice anything that hinted at the eventual reveal, being too overwhelmed by everything else that was happening. Although to her credit, my wife did notice that Luke Skywalker didn't leave footprints, though, at the time, she assumed the movie was implying that Luke was such a well-trained fighter that he was so light on his feet that he didn't leave tracks. A theory which seemed to be accurate moments later when Luke dodged Kylo Ren's swings with perfect agility. Instead, we learn that Luke's real power comes from a mastery of the Force that allows him to cross galaxies.