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.