Sources connected to Mischa tell us she was hanging over her backyard fence in West Hollywood and rambling about her mom being a witch, the world shattering ... and Ziggy Stardust. At one point she fell backward off the fence and said, "Oh my God, it's over! I feel it, and it's angry!"

She was only wearing a dress shirt and tie at the time.

Sheriff's deputies and firefighters responded to a call for a jumper or possible overdose. Law enforcement sources tell us Mischa was voluntarily transported to a hospital for mental evaluation.

We've reached out to Mischa's camp for an update on her condition, but no word back yet.

Back in 2009, she was placed on an involuntary psychiatric hold.