XKCD -- write your own damned post

I've got at least two pieces I'd like write around this: one discussing the way we approach AI research (and the innate limitations in that favored approach); the other a rant about how ddulite journalists fail to catch the important subtleties in technology. I'm sure there are more angles here so I'll throw this one out to the room. What are the examples of a slight change taking a problem from easy to nearly impossible?