Picture a $24.99 membership that bills on the 1st, with a 30-day notice clause. Notice goes in on November 4, so the December 1 charge still lands, and the last expected one is that $24.99. Then a January 1 charge appears anyway.
Because the cancel date, confirmation number and the expected last charge are written down, the follow-up is one short message with proof attached, not a phone call trying to remember what was said in November.

