I had been spending a lot of money power-leveling professions for a few of my alts when I got lucky and picked up a couple upgrades for my main character. I was low on funds and needed to grab some enchants and gems quickly so that I'd be prepared for an upcoming raid. With little time to farm, I hit the auction house and looked for some deals. When I noticed the low price of Nexus Crystals, I was reminded that my enchanter alt could transform those into Small Prismatic Shards and then again into Large Prismatic Shards. After double-checking the prices for the various materials, I concluded that I would not only save a good deal of money, but I'd make a bundle by selling extra shards. I bought all of the Nexus Crystals on the auction house and shipped them off to my enchanter. Gleefully imagining the piles of money I'd soon be raking into my coffers, I switched characters and converted the first crystal into a shard. My schemes where short-lived, sadly, when the realization that the conversion of Nexus Crystals has a cooldown, much like an alchemist's transmutes. Oh well, at least I'll have a nice stockpile of shards after about a month or so.
What are the biggest mistakes you've made?