Scholars distinguish between the pillars (arkan) of Hajj — ihram, standing at Arafah, tawaf al-ifadah, and (in the majority view) sa'i — and its lesser obligations (wajibaat), such as spending the night at Muzdalifah, stoning the Jamarat within the correct days and order, cutting the hair, and performing the farewell tawaf. Missing a pillar invalidates the Hajj; since standing at Arafah is time-bound to a single day, missing it entirely (without a valid excuse) generally means the Hajj must be repeated in a future year, and the current journey may be treated as an Umrah instead.
Missing a wajib, by contrast, does not invalidate the Hajj, but the pilgrim owes a dam — sacrificing a sheep or goat, distributed among the poor of the Haram — as expiation, drawing on the same principle found in the Quran regarding compensating for an incomplete rite (2:196), which scholars extended by analogy to cover other omitted obligations. Because circumstances (illness, crowding, missed flights) vary widely, pilgrims are strongly encouraged to consult a scholar or their Hajj mission's fatwa service promptly rather than guess.