I think you mean: In theory, files don't get corrupted in transit with TCP. In theory, they also don't get corrupted when stored in memory or on disks either.
The only reason any of these things don't cause issues in practice is checksums and error correcting codes.
No, I mean that TCP checks for corruption in transit and has the packets resent in that case. I guess you could be running a buggy implementation, but that seems unlikely with how ubiquitous TCP is.
Errors can show up any time, and usually show up between the parts that checksums correct. On the wire, TCP protects you with a (weak) checksum. Off the wire, your computer and filesystem can still screw things up. Even CPU bugs can do this.
The only reason any of these things don't cause issues in practice is checksums and error correcting codes.