Sure, the level of egregiousness matters. If a player gets a 75GP discount on their armor that's not a big concern for me as player or GM. If they are getting a combat bonus in 50% or greater of encounters, that's more of a concern.I always welcome when my mistakes are caught at the table. There is a difference between someone trying to help and someone trying to rules lawyer.
When I'm a player, I will speak up if the mistake is in favor of my character, but I am less likely to do so when it's someone else's, unless it's really egregious. I'm generally a forever DM, so I don't want to come across as a backseat DM when I do get to play.