(I know, the answer is "So? We know it's weird!")
Seriously: a co-worker's mother is visiting from Europe, and bought a laptop. Her vision is limited, so her daughter spent a long time customizing the Standard and Formatting toolbars to update the icons to be more colorful -- on the theory that when Mom calls from Europe to ask how to Print, it will be easier to say "Look for the bright-green icon" than to try to describe it otherwise.
Anyway, after spending several hours on this, she saved the document. Word ground for a long time, then closed -- and when she reopened it, her customization was gone!
We spent a bunch of time looking for it (she had also added a toolbar, which was named distinctively, so I figured we might be able to find it), to no avail (we searched for that string in recently modified files, etc.).
Eventually she redid it, then called me to do the Save. I noticed that when I went into the Save As dialog, most of the toolbar buttons in that dialog were blanked out, though I could hover on them and get tooltips, so they were still there. I got a wild hare, and *copied* her custom buttons to a new toolbar, then did a Copy/Paste for each button on that toolbar, in place, *then* closed the document, and lo! the customizations stuck.
All I can think is that she irritated Word by changing built-in buttons on a built-in toolbar, yet I (think I) redid the same process here, and the customization stuck.
Does this sound like a known issue anyone has seen?