Have you just received a tax slip you had forgotten about? Or forgotten to claim a donation receipt or other deduction? Making changes to your tax returns is simple with CRA’s online services. The CRA has a matching program which would catch these missing tax slips but this program can also result in large penalties if you are missing slips for multiple years in a row. To avoid possible penalties and delays associated with re-assessing your return, we recommend making the adjustments right away. If you have an online account with the CRA you can easily log in and include missing information. If you had HGA prepare your return then it is even simpler – just e-mail the missing information to us and consider it done!
https://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/chngrtrn-eng.html