12:04:27 AM: Error checking out submodules: Submodule 'src/lib/project' (git@github.com:owner/project.git) registered for path 'src/lib/project'
Cloning into '/opt/build/repo/src/lib/project'...
Host key verification failed.

fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@github.com:owner/project.git' into submodule path '/opt/build/repo/src/lib/project' failed
Failed to clone 'src/lib/project'. Retry scheduled
Cloning into '/opt/build/repo/src/lib/project'...
Host key verification failed.

git submoduleを使っていて、Netlify上でこんなエラーが出た時は、下記の様にするとうまくいく。

.gitmodules

[submodule "src/lib/project"]
 	path = src/lib/project
-	url = git@github.com:owner/project.git
+	url = git://github.com/owner/project.git

以前にも遭遇して、そのときに何かを参考にして解決したんだと思うけど、忘れちゃいました。公式ドキュメントだったかな？次回すぐに当たれるようにメモ。