> There's no checking to ensure you don't do this.
Why not?

Further this phrase (1) should use fewer negatives to more clearly get the point across, and (2) should stand out more. Maybe reword the two sentences to be something more concise like this:

```
WARNING: Exactly 1 CourseLocator key should index a given SlashSeparatedCourseKey. 
We provide no mechanism to enforce this assertion.
```