According to the Standish Group's Chaos Reports, the top risk factors for projects include:
Lack of User Input
Lack of Executive sponsorship
Lack of resources
Unrealistic time frame
To this I would add that you need to watch for unethical behaviors which can sabotage projects.
As far as informing the sponsor, you will need to use professional judgment and also need to understand if the sponsor is driving some of these issues. A good plan would be to confirm the observation, understand the root cause, and inform the sponsor along with a corrective plan.