Undesigned coincidences are places where one biblical document unexpectedly explains or fills in a detail from another without any apparent coordination between authors. The example given: Mark 14 records that witnesses at Jesus' trial quoted a saying about 'destroying this temple' but their testimonies disagreed — without explaining why. John 2 supplies the original context (Jesus meant his body), even though John doesn't include the trial scene. This kind of interlocking detail is characteristic of authentic historical accounts, not coordinated invention.