I don't think I can reproduce this, since it also happened when I tried it (and I'm also admin). Weird thing is, I don't remember hacking anything into SalesOrders (apart from the custom validation). Let me try in a backup install and see what happens. I'll also try and retrace my steps, since I keep a separate repo that only holds the hacks I've made to this installation.
EDIT
I have reproduced this in the online demo. Look at "SO5" or follow this link. Look at the productlines, they're empty. Only thing I did was click 'edit', select a different Quote (first clicking 'show all quotes') and then save. Got a white screen, so something went wrong. Clicked back a couple of times, then I got back into the SO, where the productlines where empty.
EDIT
I have reproduced this in the online demo. Look at "SO5" or follow this link. Look at the productlines, they're empty. Only thing I did was click 'edit', select a different Quote (first clicking 'show all quotes') and then save. Got a white screen, so something went wrong. Clicked back a couple of times, then I got back into the SO, where the productlines where empty.