There are some crazy things, but in terms of projects I've always thought that the time Sweden changed which side of the road traffic drove on has got to be up there:
At a glance, changing which side of the road to drive on seems not so big and complex...maybe its jarring, but it didn't sound so big, complex, and audacious...But then upon further thought, there are tons of big and not-so-big considerations - especially the many, many little things that might not be obvious until things were actually implemented - that would need to be addressed...so, yeah, agreed, that change in Sweden was a big doozy of a societal change! Thanks for sharing! :-)