I came across this in an internal distribution list and I bet several people have faced the same problem.
Scenario: You remove the "write" privilege on the contact entity from a certain role but the role still has full access (write, create and append to) to a related entity, lets say Activities. Now whenever a user with that role works on a contact record they won't be able to create new activities or add existing ones.... Why? This shouldn't happen right? Well, turns out it is a known issue and a fix is available for it, check this out: http://support.microsoft.com/?id=948035