In 2007, Cadbury’s Dairy Milk aired an ad featuring a gorilla playing the iconic drum solo from Phil Collins’ In the Air Tonight. Its surreal tone and apparent irrelevance to the product made it stand out from conventional advertising at the time, and it quickly became a viral sensation. The campaign helped revive the brand following a sales slump and is still fondly remembered today.

'Gorilla' is often cited as an example of mould-breaking advertising whose impact isn’t captured by traditional pre-testing. This is a misconception. The ad was tested using Millward Brown’s Link methodology and scored highly for enjoyment, exceptionally high for involvement, and on par with other confectionery ads for persuasion. The only area where it underperformed was branding. Millward Brown recommended that branding could be improved if the 360 campaign placed more emphasis on the 'A Glass and a Half Full Production’ device. Cadbury managers were initially hesitant, the strong pre-test results helped convince them to approve the ad.

The ad is reported to have increased sales of Cadbury's Dairy Milk by 9% during the campaign period which is good, but not unusual for good advertising. The trouble is, one-off hit ads can't build brand sales long-term. Cadbury tried to follow up Gorilla with more ground-breaking ads (see Airport Trucks and Eyebrows) but were unable to replicate Gorilla's success. Long-term sales are achieved by building and strengthening mental associations by using consistent elements and themes over time.

For context, this version of events is based on the original research report. The author worked at Millward Brown at the time and was asked to review the findings and conclusions before they were shared with Cadbury, due to the ad’s unconventional nature. Notably, a Marketing Week article from January 2012 incorrectly reported that the ad scored low on persuasion and high on branding. In reality, it was the branding score that was below average. The article correctly reported that 'Millward Brown had to recalibrate its own scale to measure how engaging and impactful it was... and said we should still run the ad'.
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References:
Marketing Week article in Jan 2012.
Marketing Week article in June 2018.
Debate on LinkedIn, including comments from the Cadbury's Marketing Director and Millward Brown Account Director at the time.