If you ever run across a scenario where a user advises (note: I did not say complains) that they are unable to checkout a document, or even open it for editing in SharePoint, check the following and take the following actions:
1. Attain whether the user has recently recieved a new user profile or new user account from or in Active Directory, if either are true, take the user out of the appropriate SharePoint or Active Directory groups and re-add the user to those groups in AD and in SharePoint.
2. Have user restore advanced internet explorer settings (sometimes this alone resolves the issue)
3. On list or Library where user is accessing, make a note of permissions if it is not inheriting, re-inherit, then set inheritance back to not-inheriting, if that was the case. If List or Library was inheriting, break inheritance from site and then re-inherit.
I hope this helps clear up the user who could not access thier materials and didnt complain, 🙂