Hard to think of something in the domestic game that didn't benefit city or was to the detriment of United.
Internationally in recent times it has to be Germany destroying Brazil in their own backyard.
A shout out to North Korea claiming they won games they didn't, then cutting their 7-0 defeat to Portugal short so that their people thought it only ended 4-0 and then claiming Portugal who were knocked out in the last 16 actually won it.