It was a design flaw, however if they would have bothered with product testing....
There’s only so much “what if the customer is an idiot” stuff you can do in product testing. If it works when put together properly that’s generally enough. Probably the biggest hit I’d give the product testers on this is they didn’t figure out there was an improper way of putting it together that would actually function, the best designs can’t be put together wrong.