Typically, the developers will not, in most cases, be the ones using the code except in certain specialised cases. So the last testing step before promotion to a production system should be user acceptance testing, UAT. They're generally [supposed to be] more familiar with what they expect the package to be doing. And are generally more capable at breaking things with entry flows unfamiliar to someone who doesn't use it on a day to day basis.
Do your project plans not cater for user testing? If you get users to test it, you may catch bugs earlier than post-implementation which in my world is no bad thing.