See here for an example of such a failure.
Github is returning 404 not found on the commits url, in the case of this PR the URL was https://api.github.com/repos/evergreen-ci/evergreen/pulls/899/commits.
We're not sure if this is a Github issue, but it seems to be associated with increase of 502 errors for github.
2. small delay before initial request
Creating patch documents anyway?