Ok this took a while to work out.  A client used a form based word document (Office/Word 2010) which they could protect from editing when sending out however over the last week they haven’t been able to edit the contents of the form fields.

When enabling the “Developer” view within Word the Designer mode seemed to not take effect. Normally this would switch between editing the contents or when turned on, change the size of the form.

So I went about recreating the form, copying from another document which was working, (this had me confused for quite a while) for them and stumbled across this nice little error:

Finally something to work on, then a light bulb moment (and probably something I should have started with), check what Microsoft updates had been installed.

Ah, just the 18 Office updates then….. all installed on the 11/12/14.
After ploughing through each of these uninstalling and testing I finally found the little blighter….it was….. KB2553154 for Office 2010

Found another client with Office 2007 with the same issue, for this version uninstall KB2596927

Uninstalled that and everything was back to normal. I hope this saves you a few hours and gets you out of a sticky quibble.