"Serious omission" refers to leaving out or neglecting something important or significant, which can have significant consequences or impact on a situation or outcome. Full definition
This article seeks to demonstrate that the loophole had hardly been «discovered» and that it detracts attention from more serious omissions in the scheme. (newlawjournal.co.uk)
I should be the last to assume that I have said everything that might be or ought to be said on the subject, and I am very conscious of serious omissions as well as of many shortcomings. (religion-online.org)