1 general:: When something is not awaited, as in My brother always calls when least expected, or You might know that the furnace would break down when least expected? we just had it overhauled.
American Heritage Idioms
2 general:: Phrase(s): when least expected when one does not expect something. • An old car is likely to give you trouble when least expected. • My pencil usually breaks when least expected.
McGrawhill's American Idioms And Phrasal Verbs