So, for the first time since being validated and allowed free reign to preview - I've run into the dreaded error message indicating that the HTML of my reply is invalid. After doing some experimentation, I've discovered the following:
1. The problem occurred when I copied some text from another post and pasted it into my edit-window.
2. The copied text included a smiley.
3. Deleting the smiley solved the problem and allowed the editor session to continue normally.
4. I was able to re-enter the smiley manually during the editor session - once I had erased the smiley pasted in from the original post I was copying from.
From the above I deduce:
1. Graphical elements copied in from outside the editor environment are not valid in the editor environment without editing those elements such that they are in conformance with the way those elements are formatted inside the editor environment.
2. The editor has sufficient "smarts" to know the difference between elements created natively in the editor environment and elements copied in from outside the editor environment.
3. I do not know what the differences were - I didn't get that far. Got the problem solved so quickly that it never occurred to me to look deeper. That will have to wait for the next time I bang into the problem. 
Anywho, there's my latest take on this particular Lithium bug... 