It's happened before
In the late 90s I went to a networking conference at which the skipper of a C&W (I think) cable laying ship gave us a very entertaining lecture about the various superlatives, stresses and challenges involved in putting fibres in the ocean.
Repairing broken cables involved dragging miles of cable to the surface, slicing it into two pieces and looking which side the light came from, then repeating this on the broken side until light was found again, then splicing in a new section.
He ended with a tale of being tasked to locate and repair a cable near the Canary Islands.This was something like 3 miles under water, so they put down a grapple and towed across where the cable should be. After snagging something, they lifted it and committed the slice... only to find light on both sides.
Oops. Wrong one.
They were called rather rapidly with a report of another cable failure, which they graciously fixed before doing the one they were supposed to do.
Not an exact science as someone else said!