Different machines have different specifications. It sounds obvious, yet it is something that many people who write automation scripts do not take into account. A slow machine with very little memory will run your scripts much slower than a fast machine. It will probably take it longer to render the page that you are testing. This means that the web element that has been rendered instantly on a fast machine may not actually be there for a few hundred milliseconds on a slow machine. This is long enough to cause an error in your test, but not long enough for it to be instantly obvious to the human eye.