In addition to above;
1. Direction of new valves, control valves and non return valves might be wrong.
2. LG, LT fittings or inlet outlet lines of new equipment might be misaligned.
3. Due to welding joints with the existing lines , welding slug or other wastes can chock or block the process. So flushing Will be required.
4. New pipelines, equipment and supports with existing unit will restrict the walkways and making the plant congested.
5. Another big challenge is the internal modification of process equipment. It becomes very difficult to replace,modify or add any part inside the column, vessel or reactor.
E.g in reactor where very sophisticated trays, mixing parts are used we have seen mistakes from the vendors as well. So confirm each part even small nut bolts from drawings and then go for job. These jobs preferably should be supervised by the specialist of the field.