An amonia plant in Trinidad was built according to well-established process design principles. The reliance on the tried and true, however, permitted the approval of drawings that contained an evident error, which could not be completely corrected and has left a weak point in the plant. Even though the error was clear, the drawings were approved and the parts were fabricated accordingly. This meant that the preheater's carbon steel tubes were exposed to cooled effluent gas. When the preheater was delivered, the contractor decided to correct the problem on site. Now, a plant must maintain a program of continual monitoring, relying on profile radiographic inspection and ultrasonic thickness checks to ensure that part of a key component does not fail unexpectedly.