Why did my Jekyll site suddenly stop creating paths/index.html for permalinks?

Today, I ran jekyll build on my same ol' web folder, and got a different result than ever before.

I would expect Jekyll to build paths with an index.html file in each, as specified by my _config.yml file:

permalink: /:categories/:year/:month/:day/:title

In the past, this has always resulted in a file like:

/the-category/2015/01/01/the-title/index.html

However instead of the expected behavior, today Jekyll built out each of the permalink paths ending in a .html file, like:

/the-category/2015/01/01/the-title.html

What has gone wrong?