Not testing your plan in a realistic range of scenarios. No matter how young or old your company is, there are many, many ways that an incident or disaster can unfold. Some of them happen to all companies at some point, whereas some are very specific to what your company does.
For example, a fire is a normal disaster scenario in office buildings, but a chemical spill would be a disaster scenario only found in companies handling hazardous chemicals.
No matter what your business is, it’s crucial that you list all the possible incident and disaster scenarios you could face and test your plan and playbooks for each of them. While it’s unlikely you will do this all at once, having a test every couple of months, each covering a new scenario, can get you a very long way to being prepared for anything.