You’re goddam right I told him! I’d have that on my conscience the rest of my life if I knew all that stuff and let my best friend get caught!
– Tennessee Williams
A Streetcar Named Desire, Scene 7. Stanley reveals to Stella that he told Mitch about Blanche’s dark past and promiscuity. He did so in order to protect Mitch and stop him marrying Blanche. This leads to Mitch’s breakup with Blanche, which is foreshadowed here.