On the Mixin repo's wiki tab, the article titled Introduction to Mixins Understanding Mixin Architecture has a hyperlink for code examples of Mixins:
Expected behavior would be for it to link to examples of Mixin usage in the Sponge directory.
The hyperlink currently tries to link the user to: https://github.com/SpongePowered/Sponge/tree/bleeding/src/example/java/org/spongepowered,
which, as of writing this shows the following:
Suggested fix:
If there is currently no page where examples of Mixin code can be shown, the link should be removed, if there is such a page, the hyperlink should send the user to the correct URL rather than one that produces a 404 page not found error.