Advise - project management?

The lead engineer in my department spotted a mistake in the project scope which will be costing a bomb to fix it. The project is execution which involves engineering, procurement and construction is handled by me. The scope input session is coordinated by the execution team, to be precise – by an execution planner but scope input comes from engineers from other departments, such as Technical, Inspection, Operation and so on. During the scoping stage, there is a Process Engineer who plays a role as a Project Development Manager (PMD) and me who is the Project Execution Manager (PME). The scope that went wrong was a civil scope and there was no civil engineer in the meeting. The scope was simply based on site rule and no further checks were made until the lead engineer highlighted out that civil scope is not suitable for this application. This same project has gone through multiple reviews and scope change and other department engineers are pointing these mistakes on me as execution manager. What do think my next step should be?
I may not have explained clearly, please highlight if some details are missing for you to provide an advice.