"To forget that part" means to unintentionally or purposely neglect to remember or acknowledge a specific aspect or detail of something. Full definition
Everyone seems to think all they need to do is promote to sell the book and forget the part about having a good book that will make people want to buy it. (killzoneblog.com)
We should not forget our part in this, I use the term generally for fans obviously it's not of all of them. (justarsenal.com)
It's as though someone at the factory forgot some part when putting the steering assembly together. (edmunds.com)