The only true invention of the Agile camp/movement was basically the term 'agile'. Everything else is just common knowledge created before them. As I said, clever people existed before Agile, and they were doing good work without knowing that they are following any manifesto or some other set of dogmatic prescriptions.
If you are competent and have experience (basically if you know what you are doing) you don't need to wait 6 months to see that things are not right. Of course, detecting the issue and fixing it are two different activities, but one thing is common for both: you need good people to do both.