In 1947 a British airliner disappeared shortly before it was due to land at Santiago in Chile. The last (morse code) message received from the plane was "ETA SANTIAGO 17.45 HRS STENDEC". The airport radio operator requested the plane to repeat the message twice and the same word "STENDEC" was transmitted. Then all contact was lost.
50 years later the wreckage was discovered as it had been swallowed by a glacier. No-one has ever come up with a suitable explanation for the last transmission, it matches no radio abbreviations anyone knows of but the operator described the last transmissions, repeated, as "loud and clear" and "very fast".