A bad UI can certainly doom any application. But when dealing with potentially dangerous machinery, a bad UI can kill.
Imagine yourself a guest on board the top of the line British submarine Tethis for an afternoon of jovial sailing along with 50 other guests and 53 crew, including a few caterers ready to serve refreshments.
Now the highlight of the trip arrives: the Captain orders a short dive! The dive begins… but the sub remains on the surface.
After thirty minutes everybody gets impatient. The captain decides the ship needs the weight of the water in the front torpedo tubes to break below the surface. The Captain orders tubes 5 and 6 flooded.
The torpedo officer proceeds to first verify that the tubes are dry. To do this, he checks that the outside doors of each torpedo tube are closed and not open to the sea. The levers for the outside doors all line up. He decides, of course, that all the torpedo tubes are closed to the sea.
The torpedo officer is thinking intuitively, but the UI designers had not.
When the levers all align, the doors of tubes 1,2,3, and 4 are indeed closed, but the doors of tubes 5 and 6 are very much open to the sea. The officer opens the inside doors of tubes 1 thru 4 to and finds them dry. When the officer opens the inside door of tube 5 the ocean rushes in.
The sub dives headlong instantly; it reaches the bottom in a few seconds and crashes.
Only two guests and two crewmen were able to escape the ship. All 99 others perished. UI matters, and sometimes so much that it is a matter of life and death.
This BBC video provides more details including the requisite cover up. After the documentary a rather dated, yet a bit funny, and definitely very much un-BBC discussion on UFOs follows.