Startups often make a number of key product mistakes over and over. Some that come to mind today...
1. Starting with too broad a "platform" trying to solve all categories/verticals/use-cases. E.g. "It could be anything!"
2. Starting with the wrong specific problem or niche vertical when there might be another niche that has more pain and money associated with it. E.g. Working to help gardeners get leads instead of helping lawyers (more money in the latter).
3. Starting with the wrong specific solution by not directly attacking the #1 sub-problem in the given market/use-case. E.g. Building a product to help lawyers write proposals when what they might actually need most is new business leads