A source map is a claim about two files: that this position in the built output came from that position in a source file. I had never thought of it as a claim that could be false until a stack trace sent me to a line that had nothing to do with the crash. The build had gained a step. Something small, a licence banner prepended at the end of the pipeline, after the bundler had already written the map. The bundler was right. The banner was right. The map was now a careful description of a file ...