Swapped or Changed?

A playful line-up for a familiar thing that seems different: name it, say what feels off and tick a few clues. The page lines up four suspects for free - it changed, it was swapped, you changed, memory drift - picks one and names the cheapest test that would settle it; one metered run weighs the evidence for and against each. Things only, never people. Built on the SkillSafe skill @yeachan-heo/trace (evidence-driven tracing of competing explanations).