I've been running a Challenge Modes 101 series recently to help players who are scrambling to get their transmog sets, mounts, and titles before the axe falls, and our commenter Drewbob pointed something out that I hadn't considered. The use of Invisibility Potions is a key part of the strategy for gold timers, but the recipe is a world drop in classic Azeroth. If you weren't lucky enough to get a drop while leveling, or buy it when it was more commonly available on the auction house, you may find it very difficult to get these days. You can't really farm for it, and on smaller servers, you've got to wonder if it's ever available at all. I bought my Invisibility Potion recipe back during Burning Crusade, and now that Drewbob's mentioned the sheer difficulty of finding it these days, I'm so grateful that I was always too lazy to switch professions.