Complicated vs. complex

In practice, we see that complicated work and complex work are often confused. It’s quickly assumed that we understand it, can plan the work, and that not much will change along the way. However, this always turns out to be a misconception in practice. If there are many changes but you stick to your plan, you miss the mark when delivering the result. The product no longer meets the users’ needs. Apparently, it was complex after all. Therefore, assume that most (IT) projects are complex. Customer requirements and demands will change, the problems we need to solve are abstract, and we often use new tools and technologies that create risks and uncertainties. Working based on values and principles for agility is then a good idea.

Do you have an example of a project that you initially thought was complicated but turned out to be complex in practice? What problems did this cause, and could you have prevented them by applying more agile principles?