There are a few consequences for failing to identify system requirements.
1. Strain on time and effort which can cost money for either a company or yourself (depending if you are an employer, employee, etc).
2. Additional costs for hardware. If you decide to upgrade your hardware on a later time. It will cost additional money for repairs and replacements to adjust.
3. It can cause dissatisfaction to a user if you are working as a technician (for example) which can leave a negative mark on your reputation/resume)