"You cannot delete the last block in a column".

This is an error message which sometimes appears while you are trying to move snippets about, deleting some and organising your layout.  It can be generated by trying to delete the last snippet in a column and also it can be generated by deleting the individual snippets which make up a multi-part snippet (for example two images side by side, with text below each one).

Attached to this is a full descriptive document of what happens when you get this error message and how to deal with it.