[ak217]

There are many places in your stack where data corruption can and will occur. You are correct that TLS provides payload integrity on a per-packet basis - but it doesn't protect you against silent truncation (to fight this, always declare and check content-length, or use chunked encoding). I have seen corruption occur in NIC buffers, ECC'd main memory, Xen MMU'd memory pages (yes, Xen was responsible), and multiple places in HTTP server and client stacks. None of those failures manifested until hundreds of terabytes of data had successfully gone through the system.

If you're handling data on behalf of others, it's paramount that you checksum data end-to-end. Amazon S3 allows you to do this by sending the MD5 or SHA along with the data. Google GCE allows you to do this with CRCs (which, despite what others in this thread say, are more appropriate for the task than crypto hashes, as long as you use enough bits).