Listed in "All Calendar Entries" under "Grouped Entries" event was created in 2011, recurring every second Monday ad infinitum. Once accepted, unable to delete!
First Error message, upon attempting to delete from Grouped Entries: "You must select an instance document to perform these actions."
Upon opening event, "Repeating Entries" box pops up ("This document cannot be modified. It contains the original information for a repeating meeting from when the meeting was first created. If you wish to apply changes, select a specific occurrence of the meeting.)
Clicking OK, then opening "Repeats" to "View Dates" reveals a list of dates which are not selectable from list (this event does not appear on any of them, I've looked up many), and
Attempting to delete from that opened view changes the view back to the "All Calendar Entries" list.
The person who created the entry stopped having the meeting several months after this went out, and later left the company...