One of the reasons for Costco's popularity is that its in-house brand, Kirkland Signature, is very high quality. Of course, that doesn't mean Costco knocks it out of the park with every product, and one, in particular, has become incredibly unpopular: milk.

In 2008, Costco introduced a square shape for its milk jugs, as the square containers are easier to both pack and ship. The square jugs also mean that there's no need for traditional milk crates because they are now stacked by machine — but for all the benefits to the company, customers have not been happy with the change.

The problem with the shape of the square jars is that they are significantly harder to pour than the usual jugs, thanks to the taller sides. It's a complaint that started back when the change first happened in 2008, and it was still being brought up at the most recent Annual Shareholders meeting.