Maybe this one should replace #9 or #10, but for now we'll call it #11:
11. Thou shalt not move thy plant directly from an indoor no-sun position to a full-sun outdoor position just because you believe it to be a cactus. Especially if you haven't heard of Euphorbias.
(For anyone who missed it, the previous post with the ten houseplant commandments is here.)
This particular customer not only broke #11, but they also had the plant in a glass pot without drainage (#3) and thought it was a cactus (#5).
Not their fault exactly. Nobody knows these things automatically, and it is kinda logical to put a plant that wants sun into the sun. But still, this is why there's commandment #5. It's depressing. I cleaned it up as best as I could and put it in a clay pot with drainage. It could still make it. It's a tough species. We'll see.