I have seen a Selenium test suite (for a very AJAXy application) that had random unexplainable nondeterministic failures. Actions like "wait for element with id=foobar to appear" and then the immediate next action "click on element with id=foobar" would suddenly say it couldn't find the element any more---but only sometimes and not always. People spent hours trying to find the root cause, without
![My Selenium Tests Aren't Stable!](https://cdn-ak-scissors.b.st-hatena.com/image/square/6571391198fcb45e4d7c5cab7d90e5ca2fbf4605/height=288;version=1;width=512/http%3A%2F%2F2.bp.blogspot.com%2F-7bZ5EziliZQ%2FVynIS9F7OAI%2FAAAAAAAASQ0%2FBJFntXCAntstZe6hQuo5KTrhi5Dyz9yHgCK4B%2Fs1600%2Fgooglelogo_color_200x200.png)