Myth 1: I don’t have time to test!
This is an often heard excuse when someone doesn’t want to write tests. Sure, it needs some additional time to write tests, but if you really want to write tests I am sure you will find the necessary time.
Myth 2: The customer doesn’t pay for testing!
That’s often true. At least if you say something like “I will need 40 hours for testing” to your customer. I think the best approach is to not mention “testing” as a separate topic and to treat tests and production code as a unit. So if you have to estimate the effort for a feature, you sum up the estimations for the production code and the tests, and present the result to the customer. If the customer accepts your offer, he will automatically pay for your testing efforts!
Myth 3: Testing slows me down!
At first sight this seems to be true as you have to write much more code, which means less production code per time unit. But in the long run you will notice that it pays off: you need less time to debug your code, it is easier to refactor your code, and you have to write less documentation as the tests document what your code does and how it should be used.