Pablo Hidalgo of the Lucasfilm Story Group spotted the said error in the Wampa cave scene from the 1980s "Empire Strikes Back".
If you look closely, "NEW YORK" can be seen stamped into the bottom of Luke's lightsaber. Good luck on not seeing that when you watch the star wars franchise again!
I've noted this before, but every time I watch a Star Wars movie, there's something new to notice. Especially if I'm fortunate to catch it on the big screen. This latest discovery? that 'NEW YORK' is visible on the bottom of Luke's lightsaber in EMPIRE. pic.twitter.com/Z85NPvgcVR— Pablo Hidalgo (@pablohidalgo) June 19, 2019
Visit the HuffPost for more details about this error that we now know.