This is what can happen when your joints aren’t marked somehow. This stool I made back in the 1990s, but it could have happened yesterday. It still holds me up off the floor, but…
These days I mostly use a chisel, gouge or sometimes an awl to identify which tenon goes in which mortise.
It’s good practice to help keep things straight. Here’s a chest of drawers in an article I did years ago with Trent & Alan Miller, showing the chisel-cut ID marks to identify which drawer is which. (this one I got off Chipstone’s website, photo by Gavin Ashworth)
This stuff goes on & on, you see it in house frames too. I keep planning on getting out a series of photos showing this sort of thing. There’s lots of arrangements, and sometimes you can’t make heads nor tails out of them. But the principal is the same – keep track of your joints, people.
PETER FOLLANSBEE: JOINER’S NOTES 
I once forged a 15th century arm harness (arm-armour).
Then I built the other.
Unfortunately my client actually had symmetrical but opposing limbs, left and right.
So there is still another left arm harness in my shop, as I hope for a one armed client to come by.
Very timely. I assembled a dovetailed carcass yesterday. Was about to close a joint when I looked at my marks and realized I had a piece upside down.
I do love seeing pictures of woodworking mistakes from professionals. Reminds me to back off when striving for perfection in woodworking rather than woodworking.