Check your links regularly to prevent link rot. Something which constantly amazes me is how quickly the links on a web site go out of date. This is especially true of external links, but can also occur with internal links and graphics. It's easy enough to understand why external links can be difficult to maintain. It's very simple - they are not under your control.
Web sites come and go quickly on the internet.
Sometimes sites disappear because the webmaster moved, lost interest or died. Occasionally they are removed by their ISP for legal or ethical reasons. Once in a while a web site is simply moved to a different host. The best web sites on the internet are constantly changing - almost fluid in their design. So what happens is you link to a large number of pages all over the web and a few of those each month are deleted, moved or unavailable for whatever reason.