As the combat system employed is turn-based, collisions are rare - but they may happen due to participants moving at high speed, tumbling or otherwise acting in the dark or with surprise. Additionally, combatants may be willed by others to hurl themselves at objects or persons; said objects may be invisible or the combatant may be duped by illusions, etc.
Typically, in a combat system, the situation depicted here would not occur; the player would have a top-down view of the battle field and would be able to see that a 60º turn, carried out to the end of movement, would result in a crash. Yet in real life, this sort of thing has the potential to happen all the time.
One method is to conceal part of the battlefield, then compel the character to commit to a total movement before turning the corner. Caleb would tell me he was moving up to 0705 and turning into 1004 (obscured by the building) and the move would then be resolved to show the surprise obstacle in Caleb's path.
First, Caleb would make a wisdom check, as a wise person would hesitate before turning the corner. A successful check would bring Caleb up short right at the corner - but I would also argue that said check would reduce Caleb's to a walk for 1 AP, even if there was no reason to pause. Thus, if there were no second obstacle, Caleb would run to the corner at ½ AP per hex, then slow to 1 AP, then be free to speed up again.
Supposing he does not wish to make a wisdom check, because he is pursuing someone and he willingly throws caution to the wind? At that point, Caleb is entitled to a dexterity check - with a successful check meaning that he would pull up and stop in the hex immediately ahead of the wall. A failure to make this check would mean acrash.
The same situation could occur with a tree, an unseen statue, a small cart, a large stone in the runner's path or another combatant waiting behind the corner. Caleb could feasibly slam into the back of an ordinary person waiting or working. In the case of another person, the damage from the collision would result to both combatants.
Crashing into a wooden object or surface at running speed will cause 1-6 damage. Add +1 to damage in the case of stone or metal. Objects that have notable protrusions, such as spikes, nails, pointed tree limbs and so on, then increase damage by 1-4. If the object is small enough to be tripped over, the combatant will tumble and take 1-4 damage (2-8 if the dexterity check for tumbling fails). Where a moving combatant hits a combatant that is not moving, that is also considered a crash, where both combatants suffer 1-6 damage.
In the case of collisions, where both combatants are moving towards each other, the mutual damage caused is equal to 2-12 damage. In both crashes and collisions, a d20 should be rolled. A natural 20 indicates that this damage is doubled.
Note that combatants can be deliberately crashed against, in order to slam them against or over an object, even potentially over a drop (in which case both combatants would have to make a dexterity check to avoid falling). In the case of being slamming someone into a wall, while both combatants would take damage from the collision, only one combatant would take damage from crashing into the wall. For further rules regarding attacking into an enemy hex, see charging.
See Movement in Combat