Consider a U- tube filled with water. The water in one arm of it is pushed down and left, result- the full water column will start oscillating. Now, consider an instant when the water levels in the two arms are not same and water in one arm is going up with a velocity v, water in the other arm is going down with the same velocity. Apply Bernoulli's equation to the points on the two water surfaces open to air, one on each, i.e, one point on the surface of water column in the left arm and one point on the surface of water column in the right arm. You will have pressure + half * (rho) * (v^2) + (rho ) *g * h = a similar expression for the second point. This creates all the problem. The pressures at the two points are same, so that term will cancel out. the velocities are same, so the half rho v squared term will go. the remaining term is the rho*g*h term which cannot go since the heights are different in the two columns, that means the above inequality cannot hold in such a situation. But that is the Bernoulli's thrm! What's wrong here? Is Bernoulli's equation wrong, or we have not yet learnt to apply it properly?