Answer
The assumption that linear momentum is conserved during the collision may be invalid because an impulse from the force of friction may have decreased the total linear momentum in the system during the collision.
Work Step by Step
The collision probably occurred during a certain period of time. During this period of time, the force of friction would have exerted an opposing impulse on the sliding cars which would have decreased the total linear momentum in the system.
Therefore, the assumption that linear momentum is conserved during the collision may be invalid.
