One of the first ways that I discover a design flaw is by hitting my head on it. thwack "OW." is a fairly common way for me to understand that whatever space I am in, or item that I am interacting with, was not designed for me.
According to the CDC's Anthropometric Statistics for 2007-2010, the average height for a United States man is 175.9 cm, or 69.252 inches - five feet, nine inches. In bare socks, I'm about seven inches taller than the average, plus one more in shoes, generally speaking. Eight inches is a lot of space to strike one's head on. Or, in the cases of several roller coaster over-the-shoulder restraints and the panoramic X-ray device at my dentist's office, to feel extremely uncomfortable as an object presses very strongly down on my shoulders for the duration of the experience.
A local grocery store near my work was running a Monopoly-related promotion in the store, and they had strung oversize versions of the Monopoly money across the top of the checkstand lights. Which were situated quite well above the space that most people would be passing through to put their groceries on the converyor belt. The selfie that you see, with my forehead obscured by the Monopoly money string, was taken at head height. Every time I went to that store during the promotion, I had to duck under the tightly-strung money, usually because I forgot from the last time and ran into it again.