Andrea Chang

After weeks of talks, Pfizer and Allergan have agreed on a blockbuster merger worth $150 billion.

It's a retailer's worst nightmare: trampling, shoplifting and fighting on the busiest shopping day of the year. With several high-profile Black Friday incidents in recent years, merchants and shopping centers are on high alert. Security has been significantly increased and retailers have been training sales clerks on how to deal with crowds and protect not only customers, but also employees and merchandise.