Going to the wrong airport.
Fortunately, I've never done that. Ensuring I have the right airport is something I check while booking. I narrowly missed it once, though. The valet who fetched the taxi when I was leaving my hotel from a conference in Chicago said to me, "Going to the airport?" to which I just nodded, but then he made an unwarranted assumption and said to the driver "O'Hare." I had to correct this and told the driver, "No, I'm going to Midway." Fortunately, if I hadn't heard the valet, I know Chicago well enough that I'd soon have realized if we were heading the wrong way.
When planning our trip to Rome some years back, my first attempt at an airline booking was British Airways, to change in London. But I read the fine print and noticed that while our arriving flight landed at Heathrow, the departure was from Gatwick. No, I don't think so. But a bus between those two might have been less bad than what we got from Air France, which was a walk across the De Gaulle airport terminal. What was terrible about that? The cigarette smoke.
What I have to watch out for is near-duplicate street names.