Physically, since computers work in electricity, this corresponds to a gain or drop in the amount of electrical energy associated with any given bit. Information is moved from one computer to another by sending digital signals in a stream. Any file transfer method has some chance of causing information loss. Electro-magnetic disturbances abound in the world and any of them can cause the hypothetical negative five volts to become positive five volts. This is a big problem, since any flip in the bits might dramatically change the final message.
Bad information is no less destructive today.
It is virtually impossible to perfectly shield bits in transit from electromagnetic interference. Radio waves, solar flares, and passing vehicles can all introduce electromagnetic waves that are capable of flipping bits. The problem is amplified by the fact that even relatively small amounts of information may have about a million bits in them. However, it is very rare for a computer on one network to have an uninterrupted fiber optic connection to another computer. Information that travels this way is like a commuter who takes the highway, where she can move quickly and easily, to get to a road near her home, where she must slow down because of traffic.