“Don’t cross the streams…It would be bad” – Ghostbusters, 1984
It looks like some Google Analytics properties are suffering from severe sampling issues. In one extreme example, reports show data from only 1 out of 3 (yes, THREE) sessions! The trigger: mixing hit-level and session-level dimensions with hit-level metrics, like Page + Country with Pageviews, or Event Category + Country with Unique Events.
The behavior only seems to affect some properties, and it is not obvious what the deciding factor(s) might be. It certainly is not volume, as one of my own low volume test properties seems to be affected.
Note: this also affects queries from the API with “HIGHER_PRECISION” sampling level.