The $100M headline that almost died in the backlog.
In 2012 a Microsoft engineer on the Bing ads team proposed moving a few words of ad text from the secondary line into the headline. The idea sat in the backlog for months. Nobody believed in it.
When a developer finally implemented it on a slow afternoon, revenue spiked so abnormally fast it tripped the "too good to be true" alarm — the team assumed it was a tracking bug. It wasn't.
The change lifted ad revenue by 12%. In US dollars: more than $100 million a year, in the United States alone. It became the single highest-ROI feature in Bing's history.
The lesson isn't about ad copy. It's that the team's best estimate of which ideas were worth shipping was off by orders of magnitude. Ship the test. Your intuition is not as good as you think it is.
Source: Ronny Kohavi, formerly GM, Analysis & Experimentation, Microsoft Bing