Yes, I do not mean the 2-3 shift.:D
If first has not quite let go before the second gear band takes hold it can bind for just a second and try to stop the driveline and therefore cause a backlash in the rear end that has play in it causing the clunk on the 1-2 upshift. (not necessarily enough to feel, but enough to jerk the driveline a little)
That was my thinking on it.
