How can it be missing as it's a bundled file? Are you referring to an unlikely scenario of site breakage, or am I missing something?

I think so. It happened when I was undoing theme changes made by the theme wizard.

The issue was the admin theme was pointing to a generated sprite file which was later deleted outside of Composr, leading to a broken theme image reference. Clearing the Theme Images cache fixed that.
However, it shouldn't crash so badly. I added tolerance so the Admin Zone would continue to work in this scenario.