Agree with all previous replies on this… from my experience the 3 that stand out for me on the PMI side:
1. Limited understanding of the deal success factors, value creation and a coherent PMI strategy
2. Congestion in decision making at exec level and losing sight of the value drivers
3. Underestimating culture and how things get done
An interesting area that I have seen play out recently is effort and focus being placed at the same levels regardless of the overall value potential.