Informational summary, not a religious ruling. Where the schools of law differ, both positions are named. For anything affecting the validity of your Hajj, ask a qualified scholar or your group's guide.
Why it happens: Pilgrims flying in often assume ihram is entered on landing. It is not. The miqat is crossed in the air.
What to do: Change and make the intention before boarding, or when the crew announce the crossing. If you realise afterwards, return to the miqat if you can; otherwise a dam is owed. Ask, do not guess.
Why it happens: It is sunnah, not obligatory, and the attempt injures people every single year.
What to do: Point toward it from wherever you are and say “Allahu akbar”. This is complete and it is what most scholars advise in modern crowds.
Why it happens: It is easy to lose count or start at the wrong end.
What to do: Safa first, Marwah last. Safa to Marwah is one; Marwah back to Safa is two. Seven ends at Marwah.
Why it happens: Groups sometimes move early to beat the traffic.
What to do: Stay until the sun has set. Most schools consider leaving early to require a dam.
Why it happens: The permitted window on the days of tashriq begins after midday.
What to do: Follow your group leader’s timing. Stoning before the window is not valid in most schools.
Why it happens: Almost every ordinary toiletry is scented, and pilgrims pack their usual products.
What to do: Buy unscented soap and unscented wipes before departure. A minor violation, but avoidable.
Why it happens: A lifetime’s intention, sometimes a twenty-year wait, and an unwillingness to stop.
What to do: Heat exhaustion impairs judgement before it stops you walking. Shade and fluid early is a full recovery; an hour later is a hospital. This is the single most preventable cause of death at Hajj.