Most science models that I have worked with have extremely little formal testing associated with their development and maintenance. In the more sophisticated models some form of full-system testing is done with some degree of regularity or automation. Even then, the automated full-system tests are limited to "it ran to completion", "it got the same results as yesterday", and "the results are identical on varying numbers of processors". As a consequence bugs are often discovered long after they are introduced and are typically more difficult to fix as the offending piece of code is less familiar and perhaps more solidly "embedded" in the source.
