If you didn't make a claim.. and should have...are you entitled to a refund?

That's where the taxpayer relief provisions come in handy. 

Under the 'fairness' rules, you can, for up to ten years, make a claim for a tax credit you missed, or if you overpaid your tax, request a refund.

But be careful, when you open up those years to audit, especially if they've been statute barred, you're once again open to review of the whole return for a further three years until they are statute barred again.

Form RC4288 can be helpful as its a pretty thorough review of what CRA is looking for when you ask for a fairness or relief request.

http://www.cra-arc.gc.ca/E/pbg/tf/rc4288/README.html

IC-07 sets out the guidelines for relief:
http://www.cra-arc.gc.ca/E/pub/tp/ic07-1/README.html

and for you history buffs, here's a little bit of the public relations commentary on the policy behind the relief program from December 2011:
http://www.cra-arc.gc.ca/gncy/pr/txrmcmpgnv_cmt-eng.html

And more information about refunding or reducing tax payable beyond statute barred is found here:
http://www.cra-arc.gc.ca/gncy/cmplntsdspts/rfndrdc/menu-eng.html