What can happen when you think you've seen it all

[pistonbroke]

So my brother and I headed off to the dump in my worn out old 41 half ton. Long about the third stop light on a busy four lane we noticed the old four speed was shifting hard and no matter how well I timed it or double clutched it. At the next light no matter which gear I put it in it was locked up but in neutral it appeared to be in high gear so fanning the clutch and 4.11 gears got us home and in the shop. The next morning after looking at Mac Vanpelts diagram we pulled the top off to have a look. All looked good as far as 80 plus year old spur gear transmissions go. It took us a bit to realize the the main shaft and the input shaft had somehow fused together but that is what had happened. Now the ? is why ? The oil level is good and no burnt smell but it does have a froth to it even after sitting over night, strange . Any thoughts as to why or what caused this. I'm not to worried as I have a nice 39 three speed going in with the new 255 next month but I'm curios . Tim