I learned at the 12/15/09 WDN meeting that the batch validator fails when it finds a broken link. A tool to find bad links is "Xenu's Link Sleuth". The name comes from the author's strong feelings about Scientology, but that isn't relevent to the functionality of the software.
http://home.snafu.de/tilman/xenulink.html
Sorry, OS-X users - it is for Windows only.
I've used it for years and I like what it does.
Give the starting point in complete form - "http://site.unl.edu/" or "http://site.unl.edu/subsite/"
Don't omit the "http://" or the final "/".
Unselect "Check external links".
Under "more options", chose as few functions as you need - I prefer "ordered by page".
When I want to find orphan pages, I select that by itself and do a new run.
If a troubled site causes time-outs use fewer threads. A clean site checks faster with more threads.
It helps to have a fast connection and a reasonably powerful computer. When I tried checking a 5,541 URL site using a Pentium 4 WinXP desktop with 1GB ram, it choked. A low-end core2 duo Win7 notebook with 4GB ram did it in 47 seconds.