Harry Mendelsohn in this July 13, 1915 newspaper article was not killed fighting in WWI. He was part of the CEF, but he hadn't yet left for France. No correction was printed in that paper or any of the other local papers that reported his death.
Naturally, since he died zero times in battle, his name is listed twice in the Veterans Field of Remembrance, erected in 1953 in the Baron de Hirsch de la Savane Cemetery in Montreal. This suggests to me that whatever error was made during the heat of battle not only went deeper than one erroneous telegram, but also was never corrected later, despite this man staying in the service through the end of 1919 and accumulating 120 pages of records confirming that he remained alive (if not always in the best of health).
The military records I have for him came from the Personnel Records of the First World War on the Library and Archives Canada website (under Cyrus Harry May aka Cyrus Harry Mendelsohn, regimental number 406573). His file is nearly entirely medical records -- nothing listing his units, ranks, dates, or battles (although some details can be inferred). I've looked at the files for the other Mendelsohns (with spelling variations) and similar regimental numbers in hopes of figuring out which man in the 13th Canadian Artillery actually did die in Saint Noble, Belgium on (or just before?) June 2, 1915, but no luck.
Does anyone have any idea how to get to the bottom of how this mistake happened?