My ex finally admitted she'd been carrying on a relationship on the side, in case it didn't work out with me (her reasoning). She admitted it'd been much more than she claimed, she'd even met his parents, etc. After confessing, she started self-harming (hitting herself in face repeatedly for awhile) in front of me. I called 911, very quickly realizing I might be in that scene from Fear/It's Always Sunny and that this could go very, very badly for me.
Police showed up (~4:00 AM), shining flashlights into windows, waking up my Mormon landlords upstairs.
Thankfully, she sent the police away without incident/my involvement, then left shortly thereafter. She texted around 10 AM that she'd bought me a massage as a birthday gift, which was timely and welcome. She never showed her face again until I ran into her at a birthday party maybe 6-9 months later. Next I heard, she had shaved her head and moved to Indonesia.