Reference: Aligning the Combatants, Page 21.

Sometimes it is impossible or impractical to align a charging unit because of interposing terrain, obstacles or friendly models. Troops are not permitted to move over obstacles or into new areas of difficult terrain or over another model in order to align. For example, in the diagram below a unit cannot align all of its front rank because there is a wood at the end of its flank. This might equally well have been a friendly unit, an obstacle or anything that would halt or slow down movement.

The same problem can also occur during the charge itself. The charging unit finds that it clips a friendly unit or impassable obstacle on its extreme flank before it meets the enemy. In both situations the recommended solution is the same.

The formation finds itself confronted by something it would sooner avoid. In reality troops will fall out of line to miss such things. The warrior at the end doesn't want to end up in the wall or wood, and so drops behind instead

Where an obstacle, friendly unit or new terrain impedes a charge or alignment the unit's front rank must be reduced in length to allow the line through. It is only necessary to reduce the line on the blocked flank. The whole formation is then redressed into an appropriate number of ranks.

Where a unit is forced to reduce its line like this the same penalty is imposed to that for overlong charges. It is necessary to do this to discourage players from taking a cavalier attitude towards squeezing through gaps, and to prevent any advantage being gained by creating a deeper formation. So, a unit forced to reduce its line during a charge loses the automatic right to strike first (strike in Initiative order instead), loses any weapons bonuses due to charging, and loses its entire rank bonus for combat results in the first turn of fighting.