The container failed
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It leaked on its side
The single most common complaint. A jug packed upright and shipped on its side finds every weakness in the closure, and the test that catches it is the one nobody runs.
The jug panelled
Drawn inward, usually after a temperature swing or as a solvent permeates. It looks like crush damage and is not, and the cause is the resin and the headspace rather than the courier.
The powder caked
Humidity got in. Either the seal was not as good as the bench test suggested, or dust on the rim at filling stopped it sealing at all. Both are process problems rather than container problems.
The cap seized
An alkaline product creeping into the threads and drying there welds a cap on. The customer blames the cap, and the fix is a liner and a thread that keep the product out of the joint.
The sprayer stopped working
Dip tube, seals or spring attacked by the contents, or a dried product blocking the orifice. For a stain treatment this is the whole product failing, since the sprayer is what the customer bought.
The common root
A container chosen for size and appearance, with the material and the liner inherited rather than specified. In a category this chemically active, the parts nobody specifies are the parts that fail.
Test the way it will live
Filled, closed, on its side, warm, for weeks, and in the case of a powder somewhere humid. Weigh them, open them, look at the threads and the liner. Everything above is visible in that test.
Keep a retained sample
In the production container, from every batch. A complaint eight months later cannot be diagnosed without one, and in this category eight months later is when complaints arrive.