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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