The headphones I use with my iPod died a few weeks ago and so I dug out an old pair thinking they would work just as well as the Apple headphones with the iPod. How wrong I was!
Headphones should be interchangeable as they all use a common 3.5mm TRS jack plug. It turns out though that many headphones on sale won't work properly with an iPod. In the photo you can see the jack plugs from both the Apple iPod headphones (on the left) and another pair of headphones on the right. They look the same (ignoring the colour of the cable and insulating washers) yes? Unfortunately if you use the pair on the right the iPod will keep randomly pausing. The iPod has a neat feature in that if you pull the headphones out then the music pauses. This means that as long as you don't leave the headphones connected it is hard to accidentally turn the iPod on and flatten the battery. Unfortunately just a slight movement when using the random headphones activates this feature.
If you look carefully you will see that where the metal plug meets the black plastic cable there is a small metal ring that is wider than the plug. This ring is missing from the iPod headphones. Very slight movement of the random headphones causes this ring to come in to contact with the metal case of the iPod and for some reason this triggers the pause feature. It seems to be a well discussed problem on the Internet. There are solutions but they all involve providing some sort of extra insulation around the headphone socket on the iPod -- cellotape seems to be a common suggestion. I found that pushing the jack plug through a small square of paper and then into the iPod did the trick without leaving a permanent mark on anything.
Now given that most headphones I've ever had have this extra little metal ring surely Apple knows about the problem. I would consider it a design flaw, but maybe Apple are quite happy with the problem. I'm sure that I'm not the only one who went out and bought another set of Apple headphones once I realised what was causing my music to keep on pausing!