Teams serving UK installers often mix UTC warehouse timestamps with GMT or BST product calendars. A cohort day that spans the wrong midnight shifts every activation cell without anyone shipping a feature.
SDK upgrades that change anonymous-to-known stitching create similar ghosts: returning users appear as new cohorts, and day-seven rates look healthier or worse depending on merge rules.
Before celebrating an activation lift, list SDK versions, identity rules, and timezone settings beside the chart. If those moved, treat the chart as suspect until you rebuild the cohort from raw events.