There are many examples of this.
- Non-proliferation folks who think they can actually rid the world of nukes. Will not happen.
- Does anyone seriously think they can stop human cloning, once it's technically feasible, from happening somewhere on the planet sooner or later? By fiat, by legislation, by moral appeals, etc? Will not happen. If clones can be made, clones will be made. Descriptive, not normative claim.
- AI-generated content has reached a certain point where we have to worry about a whole host of issues, most obvious being more sophisticated fakes. "Please think of potential consequences", ad-hoc restrictions, self-imposed or otherwise, are moot in the long run. It's part of our world now.