During commissioning, you discover that input point X doesn't exist, feedback wiring is missing, or the equipment type specified doesn't match what's installed. Work halts until changes are made.
Specs and prints weren't reviewed for accuracy before work started. Controls designer assumed things would be correct but never verified.
Document every difference between the original design and field reality:
DESIGN: AHU1 supply air temp sensor on discharge damper housing
AS-FOUND: No sensor—damper housing unreachable (blocked by ductwork)
RESOLUTION: Relocate sensor to return air plenum (approved by mechanical PE)