The issue he point out while using yahoo mail is described below.
If you fail to log out yahoo mail, and don't delete cookies, then next time you return you may take these actions:
1) start on login page (where asked to re-enter password)
2) click the link on on the word "MORE" that is to the left side page
3) click "mail" at top right screen.
You are now in the last person's mail (who forgot to sign out).
Michael have tested in multiple browsers/browser versions. check out the below images which cleary explain what michael want to tell to yahoo developer team.
This clearly shows that yahoo email is no longer safe in schools, universities , cafe etc. Be safe and start asking yahoo team to resolve this yahoo login issue bug soon.
Quote from Michael "I tried to tell Yahoo months ago they apparently don't give a shit. I knida wanna see how much this could/does get used lol. Maybe next time they'll care about customer service ;)"