Residential Roof Talk

So we recently completed a renovation for a local home flipper, when he first gave us the budget and had us take a look at the roof, I will be honest, it looked like a breeze, simple asphalt shingles work, scrape the old off and nail the new down. 
To our dismay, this roofs substrate had rotted all the way through. This included removing all rotted sheathing, shoring up rotted trusses and replacing all of the sub base to make sure we can adhere the new asphalt shingles properly and for years to come. 
We all make mistakes and we all miss some important things, and then there are times like this where we just dont plum see it, completely invisible to the naked eye. Guess we live and learn.