My anecdote: I was brought in to help with an AdWords campaign for an ecommerce site. I did some keyword research, ran some small campaigns to get some CTR and conversion metrics, then kicked off a bigger campain, pruning keywords as needed to ensure CTR stayed up and we didn't get a low CTR slap on the account which results in CPC skyrocketing. It was chugging along at a pretty good ROI. At that point I turned it over to the person who is named as the contact on the account with instructions on how to keep it running. A couple days later they got a call from someone at Adwords offering to "help", and they recommended turning on display ads. A few days of that and the CTR had plummeted, CPC prices had skyrocketed, and ROI tanked and the account was ruined - couldn't get a decent CPC due to the CTR hole that had been dug.

Thanks Adwords support.