While the communications between the PC and the USB chip use protocol that can in some circumstances correct for errors, then the serial data to the USB chip does not include anything that would correct errors.
Like any transfer of data it can be subject to errors.
Thanks for your reply. But I'm still not with you sir (As Mandrake says in "Dr. Strangelove".) Arduino's serial over USB behaves like an ordinary COM port, but it's an emulation. AFAIK there's no actual serial hardware involved, RS-232 or otherwise, on either end. Presumably the Windows side is a USB driver emulating a serial driver, and the Arduino side is a USB stack doing something similar but in reverse. So whether serial over USB is lossless should depend entirely on the USB implementation, no? Is the Arduino's USB code available for inspection somewhere? I'd rather not wade through the source but if I must, I must.