A few days ago, I sent an email but it wasn't deliverable. Usually, when this happens a Postmaster failure to deliver email is returned to the GMail inbox. The bug is this time the failure to deliver email went directly into the GMail spam folder.
In GMail, spam is a couple of clicks away from the inbox. Unlike some other email clients, GMail does not have a "You have spam!" alert. Probably, GMail hides access to spam because it presumes it can effectively detect spam from nonspam.
It would be useful if GMail could put Postmaster failure to deliver emails into the GMail inbox where users can see them.