When planning an event or function, what would you say is the most important information that 99% of the people forget?

In arranging corporate events, when distributing awards and shields we select dignitaries and forgot to select2nd option in case of non-availability of the1st person and second at the end of the event forget to thanks the name of the event organizers.

Showing the organizers / planners / speakers 

the exit doors for emergency purposes.

Check-list

1.BUDGET.

2.Brands guide line & objective of the event.

Preparing detailed event program with timing and alternatives action and clear missions to the Organizer team 

Usually during an event or function planning, one generally misses out -

1. A proper agenda to be communicated in advance to the attendees, say5-7 days prior.
2. To cross check whether the planning is in line with the project plan, which was created prior to event.
3. To Re-Check/Re-Confirm the finalized things.

Back up plans in case of any exigencies to ensure a smooth execution

a copy of the government permission

communicate with othres about this event. You only plan it based on your experience, but somebody else has to do it.