Like they said,the roll pin might have sheared,or perhaps the shifter thing you rotate to pull it out came loose somehow and the shifter popped out of line with the forks,(you would probably lose gear oil out of the shifter hole if that happenned)or might be the forks themselves.
I like the SM465 tranny,but it does have a few drawbacks I didnt care for when I had mine--one was the large jump from second(actually first gear,the one after granny low)to third gear---Waaaaaaa---bogggggggg--and many of them were popping out of third gear due to snap ring failure on the mainshaft,a ballbusting job to try and install one through the pto covers,usually ended in failure because you have to stretch the snapring way too far open to get it on the shaft,and it doesnt hold it good enough.I saw a thing in an old J C Whitney book that resembled a bearing splitter you use to pull ball bearings off,or a 2 peice shaft collar with allen head bolts to replace the snapring,but they vanished quickly for some reason and I havent seen them since.I'm suprised more people on here havent had their tranny's popping out of third--it was quite a common dilema in the 80's on the SM465,that and the output shaft splines wearing out on the older course spline models.I loved that granny low though! /forums/images/graemlins/crazy.gif