https://news.ycombinator.com/item?id=42937148 (works)
https://news.ycombinator.com/item?id=42937147 (fails)
(Also via email.)
This suggests an item-count or time-based sliding window to me.
As I write this, earliest accessible content is <https://news.ycombinator.com/item?id=42937674> ... and that's just become inaccessible as I'm updating this comment.