This was not "just a documentation issue." The HUD units were originally approved to be installed in combination with particular sensors based on compatibility demonstrations by the HUD vendor. Later, Boeing made a production change to use different sensors without the HUD vendor demonstrating that the HUD worked properly with the different sensors. After the issue was discovered (after delivery of many airplanes) the evaluation was done to verify the sensors were compatible with the HUD. It turned out they were, so no design change was necessary. "Paperwork" was involved, but that paperwork documented a compliance showing activity that was only done after many airplanes had already been delivered prior to the proper evaluation. While it didn't bite them in terms of an event caused by an incompatibility, that's not a trivial error.