Starting to plan too late is the most common, and the most avoidable, mistake at corporate events. Popular venues are booked up three to six months in advance. Good external speakers have full diaries. Catering with specific requirements needs lead time. If you start too late, you make all your choices based on what is still available. You don't choose based on what fits best.
A close second: not setting a clear objective. Without a concrete goal you make every choice on gut feeling. With a clear objective you make every choice based on: does this contribute to what we want to achieve? That makes the organising process faster, the choices sharper and the evaluation afterwards more meaningful.