Let's take another example. E.g. if you have a progress bar for doing a set of 5 git operations you don't know whether the 3rd might start some housekeeping. Fo

Let's take another example. E.g. if you have a progress bar for doing a set of 5 git operations you don't know whether the 3rd might start some housekeeping. For some tools (such as git) it might be possible to force this automatic housekeeping off in order to make a predicable progress bar. But in others it might not be. In some tools you may be able to predict if it will happen, and in others maybe not. Another example (from the real world this time): I make a progress bar for an iterative optimization of an design in a program. The number of iterations until the design completes can't be reliably estimated. It's not completely underterministic of course, but the complexity of making that estimation isn't just high, it's likely to be so high that it approaches the complexity of the optimization itself.