Via Clayton Cramer, I see that Eugene Volokh is asking for examples of "Unreliable Assurances" in which it was confidently stated that A would not lead to B,

examples where (1) the opponents of some proposed law, constitutional amendment, or judicial decision argued "this action will be interpreted in this particular bad way" or "this action will set a precedent that will be used to reach this particular bad result," (2) the supporters assured the public that no, of course this won't happen, and (3) some time down the line — preferably no more than 50 years, just to avoid especially hard questions of causation — the foretold result did take place, despite the supporters' reassurances. (It doesn't matter whether you like the ultimate result or not; and it also doesn't matter whether you think the assurances were sincere or not.)

Volokh has done a lot of work on the concept of the Slippery Slope (see diagram) The first three examples that spring to my to mind are

• Gun control, where everyone was repeatedly told that registration won't lead to confiscation: it led to confiscation.
• The Vietnam War , where the Left kept saying there wouldn't be a bloodbath: there was a bloodbath.
• Most of all, especially for for Vdare.com readers, the 1965 Immigration Act, in which Senator Ted Kennedy said, among other things: "Our cities will not be flooded with a million immigrants annually. "Oh, boy. Today a million immigrants flooding the cities annually would be cut in immigration.