Back in wrath, found a mechano-hog (back when they were selling for a ton of gold) on the AH for 200g. Instantly bought it out. And got a whisper a few seconds later.
Guy meant to list it for 20,000g but hit post too soon. Those happened to be the exact 3 seconds I saw it on the AH and bought it before he could cancel his low auction. He whispered me and congratulated me on the good bargain, and jokingly said he had made a huge mistake. I felt bad for him and mailed him the 19,800g difference.
Browsing the AH house and looking for good deals is still my favorite way to make gold. I have found some amazing bargains which I quickly relisted for huge profits.