For me the biggest one lately was due to miscommunication and bad assumptions. The designer was working in a redesign in their own free time, so they were usin

For me the biggest one lately was due to miscommunication and bad assumptions. The designer was working in a redesign in their own free time, so they were using this "new design" as a template for all recent mockups. The Product Manager just created tickets with the new design and was adamant that changing the design was part of the requirements. The feature itself was simple, but the redesign was significantly hard. Talking with the business person revealed that they were not even aware of the redesign and it was blocked until next year.