This rule does not appear in the rulebook but is included in the Warhammer: The Old World FAQ & Errata and is worth repeating here.

Warhammer: the Old World is a complex game of manoeuvre and counter manoeuvre between tightly formed battle lines of densely packed infantry and cavalry. It is to be expected that unusual situations will arise when units get in one another's way, interfering with movement, shooting, combat and so forth.

To deal with this, we encourage players to resolve uncertainties in a way that keeps the game flowing, or to seek the opinion of an impartial third party (at an organised event, where such situations can take on greater significance, this is the role of the event organiser or the umpires, and players should always defer to the ruling of such an official, as is right and honourable – only the most dastardly rapscallion would argue with such an exemplar of the hobby).

If such resolution is impossible, the simplest solution is to rule that the unit cannot do the thing, by which we mean it cannot make the move, cannot take the shot, and so forth. As stated in The Most Important Rule, what matters more than any rule is that players enjoy their game and that rivalries remain friendly!