We recently had a customer encounter a bug with the Organization Wide Default sharing settings.  Evidently, when you mark Activities as "Private" anyone with read access to the related record can still view a task.  It only prevents them from editing.  The customer community idea related to this issue is here:

https://success.salesforce.com/ideaview?id=08730000000BrSoAAK  

Our project team solved it with a small Visualforce page that filters the view to the owner and a selected group of profiles.  Related lists have to be treated carefully because they can still display fields that may contain sensitive information.  One nice side benefit was that the workaround did not require any Apex code.  It also leverages the default task layout so that administrators can still control the layout.

Here is the markup:

<apex:page standardController="Task">
    <apex:detail subject="{!task.id}" rendered="{!OR(task.ownerid==$User.Id, $Profile.Name=='System Administrator', $Profile.Name=='CSR Profile')}"/>
    <apex:outputPanel layout="none" rendered="{!AND(task.ownerid!=$User.Id, $Profile.Name!='System Administrator', $Profile.Name!='CSR Profile')}">
        <h1 style="font-size:1.5em">Insufficient Privileges</h1>
        <p>You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary.</p>
    </apex:outputPanel>
</apex:page>