I was unbolting the header, because I won't be able to get to the upper transmission bolts with it in place.
I had dropped the transmission mount and it was resting on my tall jack, just enough so that the weight of the engine wasn't on the header.
I decided I needed to lower the lift about 6" so that I wouldn't have to be reaching so high to get the header bolts off.
Pop!

F**ck.
Fortunately I had the jack on the cross member and not the transmission itself.
What an idiot.
WTB: One E28/535 transmission cross-member. Part # 23711128517. Will pay expedited shipping.