Having more cash on hand than you think you need. Your first round of estimates on sales is too high. And sales take longer than you think they will. Costs are also more than you think they will be. Especially if you don't follow your preliminary plan. Getting distracted by new marketing ideas that you are convinced will work. Or buying more office equipment than you need. And my personal favorite way to spend more than planned - too many printed marketing materials. That usually get thrown away. Through on some unexpected struggles - Covid19, equipment breaking or landlord forgetting to pay his bills. And you can see how extra cash is needed. Extra cash on hand is always the best thing you can do to start a business.