The issue comes from the massive fight scene near the end of the film where Rey and Kylo Ren take down Supreme Leader Snoke’s Praetorian Guard.
The hand has a blade when it disappears behind Rey’s back, but when we see the hand again, it’s empty.
Now that Star Wars: The Last Jedi is available on Blu-ray people have the chance to go over it very closely, and in doing so they’re having fun with an apparent continuity error.
Each member of the guard is armed with different weapons, but one of them starts the fight with a pair of knives, but one of them seems to vanish halfway through a skirmish with Rey.