When it is not continuous, not within the three years before filing, or not with a qualifying entity. A leave, a period as a contractor, or a stretch already worked in the United States can each interrupt it, and the interruption is usually discovered late.
Count the year backwards from the filing date
Hypothetical example: Rosalie has been plant superintendent at an East Ferris culvert and steel-fabrication company for six years, but eleven months of that were spent at the group's American plant on an earlier assignment, and she took a five-month unpaid leave the year before. The rule is one continuous year of employment with the qualifying organization abroad within the three years immediately preceding the petition. Time she spent working in the United States does not count toward the year abroad, though it may extend the three-year window in which that year can be found.
The unpaid leave is the harder fact, because whether employment continued through it depends on the arrangement rather than on how anyone describes it afterwards. Payroll records, the leave agreement and the record of employer contributions settle it. Rosalie should reconstruct that timeline before the group commits to a start date, since the remedy for a broken year is usually more time working abroad, which changes the whole schedule.