Slippery Slope Arguments in History: 1886 Supreme Court

From Boyd v. United States (1886), an early case that read the Fourth and Fifth Amendments broadly, indeed more broadly than they have been read since:

As, therefore, suits for penalties and forfeitures incurred by the commission of offences against the law, are of this quasi-criminal nature, we think that they are within the reason of criminal proceedings for all the purposes of the Fourth Amendment of the Constitution, and of that portion of the Fifth Amendment which declares that no person shall be compelled in any criminal case to be a witness against himself; and we are further of opinion that a compulsory production of the private books and papers of the owner of goods sought to be forfeited in such a suit is compelling him to be a witness against himself, within the meaning of the Fifth Amendment to the Constitution, and is the equivalent of a search and seizure—and an unreasonable search and seizure—within the meaning of the Fourth Amendment. Though the proceeding in question is divested of many of the aggravating incidents of actual search and seizure, yet, as before said, it contains their substance and essence, and effects their substantial purpose.

It may be that it is the obnoxious thing in its mildest and least repulsive form, but illegitimate and unconstitutional practices get their first footing in that way, namely, by silent approaches and slight deviations from legal modes of procedure.