In Word, type three hyphens, or three underline characters, then press Enter. With the default setup, Word will replace the characters you typed with a line that stretches across the whole text width on the page. Great if you want a divider to split up sections.

But now try deleting this line. Click below it and delete backwards. The line is immune to deletion. Try to select it. You can't.

The secret to deleting the line is understanding what Word did. The line isn't quite what it seems. In fact it's a border that is applied to the bottom of the previous paragraph.

So to delete it:

1. Click anywhere in the paragraph above where you were when you entered the three hyphens of underscores.
2. Choose Format > Borders and Shading.
3. Select the None border setting.
   
4. Click OK.

This isn’t a Photoshop job, it’s a real error message, presumably tucked away in some remote corner of Microsoft Word.

Update:

Turns out it's nothing to do with Word (more's the pity). It's a "feature" of Notepad++, which is my text editor of choice right now. I must have been closing down Notepad++ at the same time as Word.

One thing that's always irritated me is to do with tabs. Microsoft, being Americans, obviously still think in old money - i.e. I think they must use inches by default rather than centimetres, because even when you set a template to use centimetres, the default tab stops still use metric equivalents of imperial measurements. What I mean is that, although your tab ruler has quarter, half and whole centimetres marked, you try dragging a tab or an indent marker to one of these marks and it won't go. By default bullet point indents land somewhere after 0.5 cm. In fact they always go to 0.63 cm, which mystified me until I realised that 0.63 cm is the same as 0.25 of an inch. The thing that's always really annoyed me is that you can't just drag it to the 0.5 cm mark, you have to double-click it and manually change the value in the dialog box.

Well, this morning, after years of being irked by this, I finally came across the solution in a forum post by Stefan Blom, a Microsoft Word MVP (I've changed 0.5 in his original to 0.25, which I think is more useful):

Note: When you use Ctrl+T the hanging indent goes to the next available tab stop (or the default tab stops if none are set). Pressing Shift+Ctrl+T "outdents" the current hanging indent by one tab place.

The amazing thing about this is that who'd have thought that you would set the tab defaults in the Drawing Grid dialog box. No wonder I never found it before now!

I was irritated by this but I had a workaround: print to file using a PostScript printer driver and then run Adobe Distiller on the PostScript file to turn it into a PDF. This produced acceptable results, but without links (for example, you can't click on a contents list item and zap to the appropriate spot in the document).

Well I finally got round to trying to fix this. I uninstalled Acrobat 6 Professional and rebooted, uninstalled Acrobat 7 Reader and rebooted, reinstalled Acrobat 6 Professional and rebooted - but it still didn't work. No "Adobe PDF" menu or toolbar.

I Googled for the answer, which is as follows:

Ensure that PDFMaker is enabled.
Make sure that PDFMaker is enabled in the Office application:

1. In the Office application, choose Help > About Microsoft Word.

2. Click Disabled Items:

Select any PDFMaker file (.ppa, .dot or .xla) that appears in the Disabled Items list and click Enable.

3. Click Close.

4. Click OK.

This solution (and many other solutions to the same problem) comes from:
http://www.adobe.com/support/techdocs/328909.html

www.wired.com/news/mac/0,2125,63848,00.html

and a riposte by Davide Guarisco:

www.wired.com/news/rants/0,2350,63931,00.html

in which he writes:

While I confess that I wrote my Ph.D. thesis in Word 5.1, I now look back and marvel at how ignorant and naive I was at the time ... Yes, Word 5.1 may be the best version of Word ever, but my esteem of Word has gone way down. Now that I have gotten to know FrameMaker and LaTeX, I realize that Word is nothing more but a completely wrong-headed approach to word processing (and a horrible piece of software, too). Now I think that Word is like a cancer, an evil virus pervasive because of ignorance. Get rid of it!