I don't test as much as I probably should, because it seems cumbersome since I am mostly dealing with APIs like Facebook. For example, if a user revokes their Facebook OAuth app token they get an email notification about that from me, informing them that the app will no longer be able to function because of the expired token.

I am not automatically testing that, perhaps I am missing something, but automating the steps to log in to Facebook and revoke the token and then also making sure that SendGrid sent the email correctly just seem impractical.