Consider the following pseudo code:

a = 7 + b
c = 5 * d
e = 1 - f

What’s going on? Give it a sec what do you notice?

Nothing, right! We’ve simply established some relationships between variables, and that’s it. Well, not quite we’ve also specified an arbitrary ordering to these expressions, by virtue of the semantics of the language. With this trivial example, it’s easy to see that the ordering is of no consequence to the meaning of the program, but with less a less trivial example, how we choose to order things can greatly increase the complexity of the code.

This is because the person reading the code above must effectively duplicate the work of the hypothetical compiler — they must (by virtue of the definition of the language semantics) start with the assumption that the ordering specified is significant, and then by further inspection determine that it is not (in cases less trivial than the one above determining this can become very difficult). The problem here is that mistakes in this determination can lead to the introduction of very subtle and hard-to-find bugs. - Mosely & Marks (Out of The Tarpit)

It’s worth noting that this isn’t the case with all languages, just imperative languages with guarantees about the order of execution.

Another interesting/scary thing related to control-flow is concurrency.

Running a test in the presence of concurrency with a known initial state and set of inputs tells you nothing at all about what will happen the next time you run that very same test with the very same inputs and the very same starting state. . . and things can’t really get any worse than that.

So true, the complexity introduced by control flow and how things are ordered is no joke. It makes our programs very difficult to reason about and test. This example specifically points to the fact that tests are far from infallible. At best, they can increase your confidence in the behavior of a sytem i.e. that id does what you’d expect it to, but there is no certainty, and we have to constantly operate under the only assumption we can make - That we are not standing on solid ground.