Every flag should be reviewable by refs upstairs!
If it is a BLATANT mistake, like a ref calling the wrong team for a tripping penalty, the refs in the both calls down immediately, and tells the head ref to pickup the flag. I also believe BLATANT non calls, that EVERYONE at home can see, should be called by the ref upstairs. This extra official reviews the play, between plays, and makes a decision quickly to not hinder the flow of the game and to not increase the length of the game by too much. GET THE CALLS RIGHT!!!!
Also, coaches should be able to review everything, including judgement calls ( holding, pass interference, roughing the passer, etc.) Refs screw up on these calls all the time. The refs are moving fast and things are happening quickly. Sometimes they do not see what they think they see. If the refs upstairs are doing their job coaches will probably not have as many challenges.
Also, refs on the field, should never go under the hood to review a play. Let the refs upstairs, who are only focusing on penalties make the call. Refs on the field are focusing on a lot of different things. Let the refs in the both focus on the reviews AND the blatant mistakes( wrong player called for tripping etc.)
Blatantly bad calls that EVERYONE sees at home MUST be fixed!!!!! There is no excuse in the year 2024, with the technology the NFL has( camera angles everywhere etc.) to allow BLATANTLY BAD calls to go unfixed!!!!