I was looking at an old "Preview" email, and clicked a link ("http://client.c..4.com/etc."). It took me to a completely different site from the one I'd originally linked.
Do "Preview" email links expire and get reused? Can you confirm that email links sent to users never expire/recycle?
Most likely it was a preview for a campaign that you had since reimported, so the tracking links would have changed. This will only happen with previews. Once you send the actual campaign, all the links are maintained indefinitely.