Schedules change; staff join and need to be trained, or leave, taking all their knowledge and experience of the project with them, or fall ill, or take holidays. Organizations such as suppliers reorganize, merge, are taken over, and go bankrupt, usually just when you most need them. Offices are relocated, disrupting your infrastructure and carefully optimized network architecture. Back at home, people make mistakes, or forget requirements, or misinterpret the carefully worded text. Systems engineers are, after all, only human. Any of these risks could force you to compromise on the requirements and implementation.