Leader-Ender 2 project

 So, I was happily leadering and endering away while working on another project, mindlessly attaching units 2a to the finished units 1/1a/2, when I suddenly realized things weren't quite adding up:

There's supposed to be an equal number in both stacks, but even factoring in the additional thicknesses of seams, these are NOT equal!

A count and recount of what I had vs what I needed showed a unit 2a shortfall of 16. I had planned, checked, double-checked, and organized everything so carefully--how? why? when? Where did they go?

After searching around, under, and through all possible hiding places (several times) and finding absolutely no sign of the missing units, I resigned myself to cutting more of them. In the hope of finding a few more matching 3.5" squares to use up (to sew into the 2a rectangles), I opened the 3.5" bin and discovered

a neat stack of paired-up squares.

I had planned for all the units! Somewhere along the way, however, I lost track of where I was and what I was doing, and instead of sewing these together to make them into 2a units, I put them back in the 3.5" bin. Such a relief!