My personal 'favorite' is the mess that is the Irish border. The EU wanted to keep current agreements in place as a backstop, with an option for it to be extended in the event no Brexit plan was put together. Britain refused to agree on an extension option, on the grounds that they were about to develop amazing new technology what would revolutionize border security forever. They had no idea what this technology was, or how it would work, but they assured everyone that it would be amazing. Two years later, said miracle technology has yet to appear.
If nothing else, it's a reminder to not base your plans around something that doesn't even exist yet.
If nothing else, it's a reminder to not base your plans around something that doesn't even exist yet.