Just after Jesus told Peter about how he was going to die, Peter asked about John. Jesus’ reply was effectively “mind your own business, that’s between me and John”. But the way Jesus worded His response gave rise to a misinterpretation among the brethren. Jesus said “If I will that he remain...” (emphasis mine), not “I will that he remain”. Obviously, we know that John died, but in this same passage, we see that John himself is writing (v24). In other words, he had not yet died, and so this rapidly spreading saying could not be thoroughly disproven.
This passage is relevant to the practice of hermeneutics, as it teaches us how not to read Scripture. First, John tells us what Jesus did say. This is obviously important, otherwise it wouldn’t be included. But he also tells us what Jesus didn’t say. The message is clear: Don’t impose on the text what the text doesn’t impose on you. Ultimately, it is impossible to approach Scripture without some kind of preconceived bias, but these biases should have their bags packed, ready to be evicted. Always read the text as what it does say, and try not to read between the lines.