Hear Hear.

My previous employer is in the field of network and application performance management. Even with unit-test, integration-test, and some functional-test, we can't be 100% sure we've covered everything (e.g.: different network setup) due to the domain complexity.

Having said that, "testing" has always been the biggest issue for any shop that call themselves "Agile" and it's also my screening test when interviewing with any company that claim themselves "agile": "let's talk about automation test..." :)