I created a rough shape for a head and then selected the faces necessary to create an opening for the neck and hit Delete. The shape reverted back to its original cube-like shape. I immediately hit Undo and the faces returned however the head-like shape never returns! I continued to hit Undo and each time I did, the vertices returned to their original positions but the box remains.

Naturally, I had to start over, but just like before, as soon as I delete the faces, the result is the same ( as shown in figure 3).

I've never had a problem like this before. I quit Maya and opened files I recently created and performed the same operation and nothing like this happens. Could I have accidently turned on a preference setting or something for this particular Scene?

TIA