The upper layer of 3/4" subfloor was mostly rotted in the back ~3' under the toilet, and the lower layer of 3/4" subfloor (actually diagonal hardwood slats here) was also soft, but only between two joists.  After prying up the top layer and sawzalling the lower layer, Brian screwed in some 2x4s to support the first new piece of subfloor, then put a bead of Loctite on them.