agent- I also see no problem if one forgot an item & have done the same as you.
If I have a full wagon, I usually will let someone with just one or 2 items go ahead of me.
But I never can stay silent about someone saving a place in line– just standing there– while their mother (usually, but whoever) goes to do a full shopping. As someone said previously– you shop & when you’re finished, then you get on line. Derech eretz.