No matter how much prep and planning you do, every trip is bound to go a little bit awry. However, you can learn from the experience and do things differently next time.
For example, when my original flight to the UK got canceled this summer, I rebooked it for an earlier flight — only to realize on the way to the airport that I'd accidentally chosen a flight for the next day! A phone call to the airline's customer service fixed it, but when my flight back home was inevitably canceled a few hours before departure, I was sure to check the times and the dates to make sure I wouldn't accidentally leave myself stranded for a night.
Sure, I ended up doing a multi-leg grand tour of East Coast airports, but at least I had enough time to make it through customs before my connecting flight!
Another travel mistake I've made multiple times is visiting cute bookstores and buying wayyy too many books. After barely avoiding an overweight baggage fee on my last trip, I'm planning to just take pictures of the books I want and order them from my favorite local bookstore in the future.
Or maybe I'll just limit myself to a few paperbacks...