What are your top risk in Construction projects?

My list is similar

1. Not just errors in estimation but omissions of activities in the project.

2. The contract bidding climate. In a tight economy or where the project has a highly competitive bidding there is a risk of bidders who will file unfair competition complaints that can both delay progress and if the project has to be re-bid higher project costs.

Accidents.
I agree with Susan on bidding climate.
Coordination issues between trades / engineering disciplines (both, together, and often in conflict.

One more addition is half knowledge people in leadership role..

Live cables, which are not recorded in the site plans, makes it horrible for a site engineer. But the latest detection technologies have made that risk minimal.

Then, there are risks in achieving the schedule, if your project requires large scale transportation and logistics affecting timely & sequential material delivery, which occurs with many steel construction projects. I mean those projects where equipment and machinery are transported on-road for thousands of miles including different and risky terrains.

Lack of awareness of safe procedures is yet another major risk, but can be controlled through proper induction and training, however the chances cannot be completely eliminated.

WOW!
That response is very thorough of Vincent.
Let me make a suggestion, and let's deviate from the PMP Framework and get to the reality of Our Business- The successful Execution of the Construction Process. I could list the Risks here in every category, but I would rather call your attention to the Primary Focus of Risks on all Major Construction Projects- SAFETY- and specifically the avoidance of serious injuries and deaths on your Project.
Clients call me to rescue Projects in trouble- Usually related to schedule performance. Getting to the root cause and turning the Project around is relatively straightforward.
The toughest part is to create a culture of Safety on the Project, and to strictly enforce that priority. I deal with 3rd world un/semi skilled work forces (no names) whose cultures do not emphasize Safety.I have very strict rules and procedures to enforce my Safety goals to reduce those inherent risks. You should be focusing on these as well.
Ask yourself this question- Will my Project be considered successful if my EVM's are on the mark- but we "lose" some people in the pursuit of those goals?
I can tell you my answer (and the answer for the USACE and Aramco clients); The answer is NO!
The Primary/Overriding Risk on all Constructions Projects will be the Safety Culture which can only be highlighted as a top priority by the PM.

I support the view that addressing safety risk is a priority for projects, both in project execution and in the operating asset.

Regarding other project risks in Vincent's question, a proper site survey is one of the most critical as site conditions which are unforeseen can have a significant impact on projects,