Thank you both for your answers! I will offer a couple more along the same lines then:
1- GC4PVHX - Squak Mountain Blackout.
2- GC2QZYP - Tiger Mountain Blackout.
And then a question: do the finals of "series" qualify as challenge caches? A "series" is typically a set of caches (each unknown or traditional) leading to a final which is always a mystery. In order to solve and sign the final, you have to find all the others which give you clues. The final is sometimes called a "bonus", but by all due metrics it follows the definition of a challenge cache (mystery cache requiring seekers to find an associated physical cache, and requiring to find an additional set of geocaches as defined by the challenge owner).
Examples of such caches:
1- GC4PHNE - Doctor Who is 50: BONUS - The Best Idea Ever Invented.
2- GC3XRGX - A New Era. (note the description of the cache actually mentions it is a challenge).
3- GC6CWCM - Where in the 'Verse?
Thank you!