Having ordered a new server, I finally got it yesterday. I thought I’d set it up to act as secondary nameserver, backup mysql and general user-data backup but when I started backing up some of the data from the old server (via rsync over ssh) I found that every few minutes the rsync would die with the error:
Corrupted MAC on input.
This error both showed up in the shell running rsync and also in /var/log/secure on the new server as
Sep 11 16:45:11 server sshd[8254]: Disconnecting: Corrupted MAC on input.

Having logged a ticket with PoundHost (my dedicated server provider) explaining the situation, I asked if it was possible to try a different network cable and a different switch port and less than an hour later got a reply back saying both had been changed and could I check it again.
Still no luck, and still 45GB of data to copy over! Spent half the night and most of the morning restarting rsync and copying the important bits but it just wasn’t happy at all. I even found that fetching a GB of data via HTTP could get randomly corrupted! Not a good thing for a backup server.
By the time this afternoon came, I was getting desperate so updated the ticket and asked if it was possible the memory or network interface were faulty and then a short while later the machine was shutdown and inspected. They changed the RAM and put it back online and I gave it another test.
10GB copied over in one go without any errors, a large tcpdump of the rsync over ssh and it didn’t kill the normal ssh session either as it had done once before. I figure the memory was probably faulty in some way or wasn’t seated properly.
In short (if you’re finding this by Google’ing for “Corrupted MAC on input”) – check the memory, network cable, network switch or hub, and network card.