WHAT WENT WRONG in the user's tab: frames that failed to render (with the frame and the engine's own words), a preview that stopped moving (and at which frame, and why), compositions that would not load, clips that will not decode, missing footage, failed exports, a page the browser killed — plus warnings (notes about unread styles and the like). Call it FIRST when the user says something is broken or looks wrong. Works while the tab is hidden or closed: the tab sends what it saw to the session as it happens. current belongs to the composition as it is now; earlier counts problems an edit has since left behind. New problems also arrive on their own: any tool result may carry a problems list of what is new since you last heard. An empty list only covers what the tab has actually played — run check_playback for a full pass.