If I mistype my username or password when logging in to Stack Exchange, the red dialogue box tells me which was incorrect.
If an attacker correctly guesses an email address present in the database and submits a bad password, the dialogue helpfully informs them they've typed an existing email address, but that the password is incorrect.
References as to why this is a bad thing, from Sec.SE:
Can we fix this security bug, or at least have an explanation why it won't be fixed?
Yes, this is a similar question to Don't give away incorrect password/username info, but that question is merely seeking discussion.
It does not have an accepted answer, no employees or Community Mods took notice, and it's tagged with discussion, not bug. Moreover, it doesn't have status-completed / status-declined / status-bydesign / status-anything-else so that question does not fulfill this bug report.