Which of the following is NOT an indicator of potential building instability?

Intact windows serve as an indication of structural integrity rather than instability. When windows remain unbroken and intact during a fire or other structural stresses, it suggests that the overall framework of the building is still sound. In contrast, falling bricks, interior collapse, and sagging roofs are all clear signs of potential instability. These indicators suggest that the building may be under significant stress or has been compromised, which could lead to further structural failure. Therefore, the presence of intact windows among these options highlights their role as a sign of stability, making them the correct choice for identifying what is NOT an indicator of potential building instability.