One of my clients went live with their SharePoint 2010 environment today. We discovered that a certain group of users couldn’t access their default home page. SharePoint responded with “Access Denied” and the usual “sign in as another user” or “request access” response.
When we used the nifty “Check Access” function it confirmed that the end users really did have access. Yet, they could not get to the page.
I followed a lot of roads to various dead ends until I decided to compare the web parts on the broken page against a similar working page. I did that by putting the page in maintenance mode by adding “?contents=1” to the page. So, it looked like “http://server/subsite/subsite/default.aspx?contents=1”.
This showed me two web parts named “Error” with a description like “Error” on the broken page. I didn’t think to take a screen cap at the time.
I removed them and that solved the problem.
I’ve seen a question like this come up on the forums in the past and I was extremely skeptical about the poster’s insistence that he had security set up properly. I *know* I had security set up right 
Next time, I’ll be more open and less skeptical.
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Paul, we have exactly the same issue going on at a client site right now. We have a very simple 3-tier portal on a single site collection in SP2010 Enterprise. Inheritance is currently “on” for all 3 levels. Users in the Members built-in group (contribute rights) can “go anywhere”. If you take one of those users out and give contribute rights directly OR put them into a custom SP group with contribute rights, they get “Access Denied” on default.aspx down on the 3rd level team site. Using the debug mode noted above reveals *no* broken web parts on that page. It seems as if users *outside* of a built-in group only have “limited access” rights. Have you ever seen this before? Full inheritance seems broken. Any ideas?
I know this is ancient. Sorry for never responding. I hope you figured out your issue.