Assuming you are talking about Oxycontin, the problem there was that there was no expert consensus - in fact there was no information at all on addictiveness - but the FDA still allowed that language to be used in the promotional literature. The guy who approved it ended up working for Purdue a few months after leaving the FDA.

The story of Purdue Pharma and the Sackler family definitely exposes issues with the FDA, but I'm not sure it's a good example of expert consensus getting it wrong. (Not to say that expert consensus is always right.)