If you have both a left and right float affecting some content, and the floats do not have much room in-between, content can run into the shape area of the second float. The codepen with triangles shows the right side of a word or two impinging on the second float. This codepen shows multiple words in one or more lines running through the second float:
In both codepens, you need to narrow the window until just before the right float clears the left.
Created attachment 244273 [details]
Minimal test case, requires Ahem font
Created attachment 244357 [details]
Comment on attachment 244357 [details]
Clearing flags on attachment: 244357
Committed r178192: <http://trac.webkit.org/changeset/178192>
All reviewed patches have been landed. Closing bug.