In 2014, researchers at the US Food and Drug Administration found a box in one of their freezers. They opened it and discovered number of vials with hard to read labels, and six that said they contained variola - more commonly known as the smallpox virus. The freezer had been transferred to the FDA from a lab at the National Institutes of Health some time before the box was discovered. The vials were dated from the 1950s.
Any chemist will tell you that keeping the contents of their laboratory freezers up to date is very difficult, especially with many researchers at all levels using the same freezer for similar, and not so similar, projects. This was likely how these vials ended up left behind in a box of samples in a government freezer. While most errors in inventory control on laboratory freezers likely wouldn't be a terrorism risk, this example shows perhaps the worst results that can occur with lackadaisical record keeping and demonstrates the importance of keeping good notes about where things are.
To help yourself and your research team keep better track of your samples, work together to implement a system for noting what goes in and out of the freezer. When possible, avoid keeping many different samples together in unmarked boxes. Set calendar reminders for responsible parties to do inventory on the freezer at predetermined intervals. Some freezers contain a great deal of material, so this can be a large undertaking. But doing it on a regular basis will prevent dangerous substances from languishing in the back of your freezer for sixty years.
Read more about keeping good notes on the Documentation page of the Data Management Best Practices guide.