"Two lines perpendicular to the same line are parallel to one another." is a first-year geometry theorem. No one was watching me in the store, other than the security cameras, so, I pulled three identical squares from the rack and spread them out on the floor. Their labels did not wrap around the square, which would have compromised my test but the labels were only stuck on the surface of the squares. I used one square as a straightedge. I slid the other two squares against this side. Then I slid them toward each other. I was careful to be certain the squares rested firmly against the straightedge. If the squares are really square, the vertical edges should meet along their length. Notice the gap between the edges where the two squares meet each other, especially how it forms a shallow "V" that becomes wider at the upper part of the photo. These squares are not square. See the second photo from a close-up photo of the gap.
I had checked framing squares this way in two other stores, but had not thought to take a photo. These squares are shorter than a framing square. This particular store had only one framing square on the rack, so I chose to test these shorter squares. The framing squares I checked in other stores all had a "V" gap, too; but, the gap on the squares shown here was the most severe.
One lesson learned is that squares off the rack in all price ranges may be accurate enough for framing houses with 2 x 4s, but those I checked are not good for precise work, like making furniture, without making adjustments with an anvil and a ball peen hammer. (This method for adjusting a square makes a dimple that brings the legs of the square nearer to each other or pushes them apart, depending on where the dimple is placed in the corner.)