Startups commonly make mistakes with intellectual property (IP) that can be costly. Some key IP mistakes startups make include failing to patent products and innovations, not securing ownership of IP developed before incorporation, using IP from previous employers, choosing weak trademarks, and uncontrolled use of open source software in products. These mistakes can result in losing ownership of IP, costly lawsuits, delays, and reduced acquisition values. It is important for startups to seek expert IP advice to avoid costly IP issues in the future.