Tested approved and failed in production; why implementation is hard

To many projects are tested and verified, signed off on by users and rolled to production with nothing but issues, why? and what can IT do about it. There are several methods for moving new products to production, like turning the test bed into Production, (though one needs to know this prior to the build out of the environment, else pitfalls will arise). 

So, planning is the easy answer to this but how do you know to plan for scale and sustainability? Trust in your product and IT team. For these are the folks that, 'know' the environment and product lifecycle.