I've been doing the ol' pen and paper since I started working. I know I completely messed up filling them out the first year or two. e.g., Not claiming the basic personal amount.
In the end, after I sent it in, they always corrected the mistakes on the notice of assessment. Didn't claim basic personal amount, they fixed it. Didn't add properly, they fixed that too. I do wonder sometimes why I don't just send in a blank form with a T4 haha.
Someday we may need to activate the halo structure off Deerfoot and destroy the North East.