this is a duplicate of the issue entry i've submitted here: http://umbraco.codeplex.com/WorkItem/View.aspx?WorkItemId=25892

i've got a feeling, this forum is more visited one.

So the problem.

Umbraco: 4.0.3

asp: 2.0

windows: several, iis: 6, 7

How to reproduce the subject case:
1. Add a 3x3 table to a page.
2. Put some text in cells 1:1, 2:2, 3:3.
3. Click "edit HTML" then update
Now you should see a table with just one column.

and a fix for this is to edit validElements entry in tinyMceConfig.config by replacing
-td[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor|scope]

with

#td[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor|scope]

# flag lets tags to be empty. In this case tags <td></td> will be kept.

This is a serious issue and hopefully next release will include this by default.

Btw. For not to feed the entropy, here's another issue with proposed solution and this one's still unresolved but not critical since one can and should use anything-but-IE, imho.