I think Tim Staples referred to this story during a Catholic Answers Live Q&A in the week.
Unfortunately, I didn’t pay attention to the name of the pope.
Apparently, while he was a Bishop, he supported a particular heresy. A queen maneuvered to get him elected (with the old pope being poisoned in all likelihood), and the new guy was made pope as the two planned. However, after his election, he changed his mind, and never formally taught the heresy.
Something like that. :o
Here are the popes that were murdered, so this might narrow things down…just look for each of their immediate successors.
John VIII (872-882): Poisoned and clubbed to death
Adrian III, St. (884-885): Rumored poisoned
Stephen VI (896-897): Strangled
Leo V (903): Murdered
John X (914-928): Suffocated under a pillow
Stephen VII (VIII) (928-931): Possibly murdered
Stephen VIII (IX) (939-942): Mutilated and died from injuries
John XII (955-964): Suffered a stroke while with a mistress
or murdered by an outraged husband
Benedict VI (973-974): Strangled by a priest
John XIV (983-984): Starved to death or poisoned
Gregory V (996-999): Rumored poisoned, probably malaria
Sergius IV (1009-1012): Possibly murdered
Clement II (1046-1047): Rumored poisoned
Damasus II (1048): Rumored murdered
Boniface VIII (1294-1303): Died from abuse received while a
captive of the French in Anagni