Forgot to include a T5 in my tax filing, but the CRA caught my mistake and adjusted for it. Do I still need to file for an adjustment or is no action needed?

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You only need to adjust the return if you disagree with the Canada Revenue Agency's assessment. Canada Revenue Agency (CRA) receives a copy of all slips. They check to make sure a taxpayer filed a return with all the slips they have  and will amend the tax return to add slips the taxpayer missed. So, CRA has done the adjustment for you. 

It is very important to add all your slips to the original filing. CRA can penalize taxpayers who repeatedly miss adding all slips to their tax return. If you receive additional slips after filing, adjust the return before CRA catches it.

Below is a link to the CRA with a discussion of their penalties, See the Failure to report income penalty section:

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/interest-penalties.html

Hope this helps