In 2007, Nassim Nicholas Taleb wrote a very influential book called The Black Swan - the title referencing to the idea that no matter how many white swans you see, you can never infer from them the existence of a (much rarer) black swan. The book was concerned with extreme events, particularly in the realm of investing, and how their likelihoods are so commonly underestimated. Taleb argues, among other things, that investors work under a flawed assumption that stock prices movements are normally distributed, so extreme market movements are unlikely. They are unlikely, but much less than expected, because traders do not always act rationally, and outside events can have a huge impact. He describes these unexpected triggers as black swans - surprising events, with a large impact.

So what does this have to do with software development?

Plenty, when it comes to estimating, planning and delivering projects. Most people have an assumption that a software project is, at heart, a reasonably predictable thing, and to varying degrees they're correct - based on experience, seasoned developers and project managers can usually come up with fairly decent estimates of how long a project will take. However, they all attribute incorrectly low probabilities to 'extreme' events occurring, which makes software projects more risky than they need to be. This is exacerbated by the speed of web projects, which, particularly in languages like PHP, are shorter and faster moving than more traditional projects. This means absolutely smaller events can have relatively larger impacts on the schedule.