A base too thin for the soil[2, 31]
On clay or silt, a base designed for sand leaves wet fine soil freezing right under the stone. The patio heaves in places from the first winter (frost heave), then sinks into dips at thaw.
Pick the right soil class in "Your project" (or "I don't know", treated as clay), build the full calculated thickness in compacted lifts and lay a geotextile on fine soil.

