Here’s an error with a very distinguished heritage.
When in 1 Corinthians 13:12 Paul tries to express the imperfection of mortal understanding, he compares our earthly vision to the dim and wavery view reflected by a typical Roman-era polished bronze mirror. Unfortunately, the classic King James translation rendered his metaphor rather confusingly as “For now we see through a glass, darkly.” By the time of the Renaissance, mirrors were made of glass and so it was natural for the translators to call the mirror a “glass,” though by so doing they obscured Paul’s point. Why they should have used “through” rather than the more logical “in” is unclear, but it has made many people think that the image is of looking through some kind of magical glass mirror like that in Lewis Carroll’s Through the Looking Glass.
Although most other translations use more accurate phrasing (“as in a mirror,” “a blurred image in a mirror,” etc.), the King James is so influential that its misleading rendering of the verse is overwhelmingly more popular than the more accurate ones. It’s not really an error to quote the KJV, but if you use the image, don’t make the mistake of suggesting it has to do with a dirty window rather than a dim mirror.