Yes, it is gone from the list. All it shows next to "Explode" is a cloud with a question mark.
I noticed that if I highlight "Erase" in the Commands List box, and look at the Image information in the Properties box, the Small & Large image shows a file name of RCDATA_16_ERASE.
Highlighting "Explode" in the Commands List box, and looking at the Image information in the Properties box, the Small & Large image is showing a path to AutoCAD LT 2008\R13\enu\Support\Icons\Tools.BMP
Apparently I've overwritten the path to the original button image, but can't figure out how to point it to the original image of RCDATA_16_EXPLODE.