Wire lines propagate signal at about 1nS/foot. To null a transmission
line after a signal step requires a wavefront round-trip (with near-
perfect termination). If you have "one in the pipe" and launch a
second transition, the "symbols" will collide and add / oppose at
the receiver, modulating the "eye" edges.
I'd say at 1GBPS you would be likely to see this at any length
over 6" or so. Most line interface parts for high speed and long
wire have some sort of pre-emphasis and try to adjust it
based on signal history, or so I gather, to correct that effect.