In case of cast restoration, if the contact is open, what should be done to prevent the complications?
If the contact is open (short of touching the adjacent tooth), a new contact area must be soldered to the casting. An open contact is best detected by visual inspection with the aid of the mouth mirror. The region must be isolated with cotton rolls and dried with the air syringe. Selection of the proper horizontal viewing angle usually discloses the spaces between teeth. Such an open contact permits the passage of food, which affects and irritates the interproximal gingiva.
Sturdevant operative dentistry  7th Edition, Page No:e147
A new casting should be done
A new contact area must be soldered to the casting
The cavity preparation should be refined further
Leave it as it is, no complications occur