Continuing with your Hansel and Gretel metaphor, the first time the candy turned sour for me was Goog-411. It was a service (at least over here in the US) that allowed you to look up local businesses via voice prompts over a phone call. When the service was still active, calling 411 via a mobile phone had a per-call cost, so a free lookup service seemed great.

I found out later that Goog411 was created to primarily train on the voice prompts from the users of the service. Left a really bad taste.