A few nice 3D Printer images I found:
Image by ehud42
I needed a support out at the corner of a print. I did not see this happen, so I’m speculating.
It looks like the support was never used, and that somehow my printer managed to turn a 90 degree corner in the middle of a "bridge".
Someone else noted that what most likely happened (and reviewing the gcode would support this) is that the first layer above the support barely touched the tower (but did – just enough to enable the corner). Then as the layers were added and cooled, the material shrunk (as it’s prone to do) pulling away from the support tower.