A client's site recently had 600+ (and then 1200+) broken links –most internal– reported with no content changes on their part. The reason is listed as "Connection Failed", and to my knowledge there haven't been any server changes since the problem cropped up a few days ago.
The issue started with WP 3.3.1 and I upgraded to 3.4, and the issue was still present.
I've manually marked the links as not broken, but there have been a number of additional broken links that came up even after I just deleted the last of the 55 pages. I've deleted the remaining ones, and then more just keep coming back.
I saw a few people who suggest the problem might be internal to the server or a DNS issue, but the site has been operational with no DNS changes for about 5 months now.
If anyone has an idea of what might be happening, that'd be great.