A problem that developers frequently start encountering with fairly big projects that need to involve third-party services, networking, and concurrency is that it becomes hard to ensure that tests that integrate many components behave in a predictable way.
Sometimes, tests might fail just because a component responded later than usual or a thread moved forward before another one. Those are things our tests should be designed to prevent and avoid by making sure the test execution is fully predictable, but sometimes it's not easy to notice that we are testing something that exhibits unstable behavior.
For example, you might be writing an end-to-end test where you are loading a web page to click a button, but at the time you try to click the button, the button itself might not have appeared yet.
Those kinds of tests that sometimes fail randomly are called "flaky" and are usually caused by a piece of the system that is not under the control...