Wrong hotel: Did that in April when I booked the wrong Hilton Garden Inn in Toronto. Fortunately while waiting for my outbound flight I decided to google around the maps a bit to check on food options for lunch when I arrived, opened my email reservation confirmation, clicked on the "local info" link, and the map looked all different! Fortunately it was not all that far out of the way but sure glad I didn't find out at the hotel desk.
Oh, with family in tow checking into the Andaz near Liverpool St. Station in London a few years ago, they kept having trouble finding my award-based reservation... sat down to wait (very nice service there) and it turned out I had made it for the following week! They very graciously managed to move me over and it was a really wonderful stay.
But the worst was just a few weeks ago. I was going to Seattle for a conference. It is always the first full week of September. I booked the hotel from the conference web site, but somehow I didn't notice that it was a week later this year. So my plane tix (on two carriers, unfortunately) were for the wrong week. Fortunately I asked a colleague the day before our flight what time she was getting in, and when she said "next Saturday" I realized my mistake. I came so close to getting all the way out to Seattle without a reservation and no reason to be there (not that I dislike visiting Seattle!)