Is there any economic circumstance that'll cause a simultaneous and sustained selloff in the following? S&P500 Gold US Government Bonds
Had the US been forced into default by the dumbasses in Congress, this could easily have happened. The SP 500, obviously. Treasuries, easy to make the case. For gold, not at first intuitive, but it would have involved a massive drain in global liquidity on the order of what happened in the fall of 2008, and we already know that brought gold down.
All those things trade vs "cash". If you have a proper flight to liqudity, driven by redemptions and an overwhelming desire to stuff dollars under mattresses, all three can sell off. That's partly what went on in 2008.