Madelyn Rancourt spotted a drinks canister and a water bottle in the background of one scene, which she recorded and posted to the social networking site TikTok in a video viewed more than 91,000 times.
Because there are no timestamps on TikTok it is not clear when the video was posted, although it started to go viral this week.
The scene shows the March family — including lead character Jo March, played by Saoirse Ronan, meeting their neighbor Laurie in his home. A plastic water bottle and a hydro flask can be seen on a table in the background.
After Rancourt posted the video other fans took to social media to say they had also spotted it.
The actors and director don’t appear to have spotted the additional set prop, even though they directly discuss the scene.
Sony Pictures did not respond to CNN’s request for comment.