It leaked in the wash bag

The closure was the convenient one

A flip top or a spout, chosen because customers prefer them, holding the same pressure difference across a much smaller sealing area than a lined screw cap would. That is the first decision.

The bottle was filled to the top

Which feels generous and leaves no room for the trapped air to expand. More headspace means less force on the closure, and filling below the shoulder is free.

The cap was run down by hand

Torque applied by fingers varies by a large factor between people and between days. An under-torqued cap on a pressure-stressed thin liquid is a leak waiting for a climb.

And it was laid on its side

Which is how a wash bag travels. A bottle upright keeps the liquid off the closure and a bottle on its side does the opposite, so the upright bench test proves nothing relevant.

The damage is to everything around it

Shampoo through a packed bag ruins clothes and documents, which is why this is remembered as a product failure rather than a minor one. The cost to the customer is far more than the bottle.

A bag inside the bag is the cheap mitigation

And worth saying in the copy rather than pretending the closure is perfect. Customers already do it; acknowledging it reads as honest rather than as an admission.

Test it the way it travels

Filled to the intended level, capped with a measured torque, laid on its side, at reduced external pressure. Four conditions, and all four have to be in the test for it to mean anything.

Fix the cheap one first

Headspace and torque cost nothing and address most of it, and they are both under your control at the filling step. Changing the closure is the expensive answer and it is often not the one needed.

References