If you have ever had to do QA on your product, you can relate to the nagging sense of dread whenever a manager walks past your door those weeks. Or you hear the customer, who speaks loud enough the office can all hear, explaining their latest error on speakerphone.
And unless you are brilliant, you’ve even had a problem in the second round of testing that worked in the first place.
Or perhaps, you are a test driven developer, but every so often the browser just does not want to care that your unit tests all pass.
If this is you, investigate selenium. Or hang around a while for a proper series of mini tutorials