You may be familiar with the Bus Factor concept (in short, the bus factor is the number of people in your team that need to get hit by a bus before your project can’t go on).
This metric is typically applied to the development team to make sure that information gets shared among all members but we usually forget this same metric should be applied to all the other areas of your startup. Even for small startups, you should make sure that you have a bus factor of 2 in every single important aspect of the company. Some may be obvious (e.g. access to the bank accounts) while others may not but are equally important (i.e. key contacts in popular websites that could talk about your product).
Remember nobody is immune to buses. Be protected!