crosstalk
On an old telephone line you could sometimes faintly hear another conversation bleeding into yours. That leakage of a signal from one channel into a neighbouring one is crosstalk, and it happens on circuit boards and inside cables for the same reason: two wires running side by side are coupled, electrically and magnetically, whether you want them to be or not. A busy, fast-switching trace whispers a faint copy of itself onto its quiet neighbour.
Crosstalk is simply capacitive and inductive coupling between adjacent conductors, working together. The fast-changing voltage on the aggressor pushes current through the stray capacitance onto the victim, while the aggressor's changing current induces a voltage through their mutual inductance. The amount grows with how long the two wires run parallel, how close together they are, how fast the aggressor's edges are, and how high the victim's impedance is. Halving the spacing roughly doubles the coupling; doubling the parallel run length roughly doubles it too. A particular menace is a fast clock line running alongside a sensitive analog or reset line.
You tame crosstalk by separating the wires, shortening their parallel run, slowing any edges that need not be fast, and slipping a grounded guard trace between aggressor and victim to catch the stray field. A ground plane beneath the traces helps enormously by shrinking each signal's return loop. Honest reality: in dense boards and ribbon cables you cannot eliminate crosstalk, only budget it, keeping it safely below the victim's noise margin. Grounding every other wire in a ribbon cable, for instance, is a cheap, standard way to roughly halve it.
In a ribbon cable, a fast clock on one wire couples onto the data wire beside it. Dedicating every other conductor to ground (signal, ground, signal, ground) puts a grounded wire between each pair and cuts the crosstalk dramatically.
A grounded wire between neighbours catches the field before it crosses.
Crosstalk depends on edge speed, not on clock frequency alone — a slow clock with very sharp edges still couples hard. Slowing down unnecessarily fast edges is often the easiest fix.