While playing Shipwrecked, I found a blueprint for an Obsidian Machete. I used it, but did not learn the recipe.
Since obsidian items are similar to Ancient-tier, I'm assuming that blueprints for them are unintentional.
Steps to Reproduce
I found a "Message In A Bottle", dug up the following chest, and received an Obsidian Machete blueprint.
This issue has not been confirmed by a developer yet.