When my bank merged and acquired 2 banks 2 years back, we had experienced a nightmare on the system integration side. There were lot of common customers who had account and banking facilities in both banks. Experienced few hiccups in cases when the banking facilities approved for both banks start getting duplicated in the system instead of getting consolidated, considering only one merged bank and system for the same customers going forward. That indirectly shows inflated exposure for those customers in our system which eventually impacted the security structure and risk rating etc. It took almost 2 months to fix that system issue at the time of integration and to make customers satisfied, until then we had to ensure that no additional loans / funds were released to the customer looking into the erroneous systems.