Here is a patch that demonstrates some oddities when trying to do a shared context while doing a texture readback from that shared context. The upshot of the below patch is that jit.gl.texture is will convert to a jit.matrix only in fairly specific circumstances. More detail is in the patch.
I haven't gone so far as to test on Jitter 1.6 on a PPC or MacIntel or XP, so I can't definitely say it's peculiar to 1.53. Boiling down the mysteries to a reproducable state on one machine was enough! If you need me to test on those platforms, I can.
tested on OSX10.39, max 4.57,jitter 1.53b2, G4/350 with 800mhz card,ATI radeon 9800 graphics card