A genuinely unique system, found nowhere else
Under SDCL § 20-9-2, contributory negligence does not bar a plaintiff's recovery when that negligence was "slight" in comparison with the negligence of the defendant — but in that case, damages are reduced in proportion to the plaintiff's fault. South Dakota is described repeatedly, and accurately, as the only state in the nation using this particular standard, rather than a percentage-based pure or modified comparative system.
A real case: 30% fault, barred as a matter of law
Here's the case that defines the practical boundary. In Wood v. City of Crooks (1997), a jury found the plaintiff 30% at fault, with the defendants collectively 70% at fault. The South Dakota Supreme Court held, as a matter of law, that 30% contributory negligence was more than slight compared to the defendants' combined negligence — barring the plaintiff's recovery entirely. In most other comparative negligence states, that same 30% finding would simply reduce a plaintiff's award by 30%, not eliminate it.
No fixed percentage, but real patterns
South Dakota's statute doesn't set a specific percentage threshold for what counts as "slight." Case law and practical guidance have generally treated roughly 10% to 20% fault as slight, while 30% or more has been found, at least in Wood, to cross the line into "more than slight" — barring recovery entirely rather than merely reducing it. See our full guide to how fault is assessed in South Dakota for more on this, and a real 2015 legislative effort to address the uncertainty this standard creates.