Even a dead end tells you which door to open next.
When you Know Things and roll a 6-, you may choose to not mark XP. If you don't mark XP, the worst that happens is that the GM tells you nothing interesting or useful about the subject, but instead tells you how you could learn more.
This work includes material from Stonetop by Jeremy Strandberg, published by Lampblack & Brimstone, licensed under the Creative Commons Attribution-ShareAlike 4.0 International License.