According to official errata, "The appendix for accidental charges on page 262 of the Warhammer rulebook has led to some dubious tactics and confusion, and should therefore be ignored."

Inasmuch, it has been included here for completeness and posterity.

In the hurly-burly of combat you sometimes need to charge across the front of one enemy unit to reach another. Normally the only problem this gives you is the fact that you can end up with your flank exposed to an enemy charge in the following turn. However, sometimes our unit ends up touching, or very nearly touching, another enemy unit with its side. It is obviously wrong to ignore this second unit when it comes to working out the combat, but what happens? Do they get a charge reaction such as stand & shoot?

In Fig. 1, unit X is free to declare a charge on unit A as it can clearly see it and it is within the charge arc.

In Fig. 2a, the cavalry charge in (remembering to bring as many models into contact as possible) and realign themselves on the target. This brings them within a fraction of an inch of the front of a rather surprised unit B.

From the point of view of unit B the enemy's charge goes thundering past their noses, but instead of disappearing into the distance, the unfortunate foe halts with their exposed flank a mere sword's length away. It's too good an opportunity to miss! Unit B can't believe its luck and charges into contact.

Note that unit B does not get a charge reaction in this case but does count as charging and all the normal rules apply.

In the subsequent mêlée both units X and B count as charging so fight in Initiative order. Unit A strikes after the chargers as usual.

Note that this extra move only applies if a charging unit ends up within 1" of the front of an enemy unit, ie, somewhere they could be charged by it.

Fig. 2b shows that sometimes you can follow the letter (if not the spirit) of the rules and bring the maximum number of models in while still staying over 1" away from unit B. In this case the cavalry charge in and the combat proceeds as usual. Unit B has to hope the combat remains where it is so they can charge the cavalry's flank in the following turn.

Note that as you aren't allowed to measure before you declare a charge it is extremely difficult to do this deliberately and rightly so as it is almost as bad as the evil clipping!).