Heavyweights - Whoops Lars, Better Learn a Male from a Female
Fast forward to the part where Lars is weighing the kids for first time. When Sims gets up and Lars tell him congratulations, he says " Congratulations MRS. Sims, you are the fattest boy in camp." He is a boy not a girl.
This slip-up could be an actual slip-up, or it can be one of two things:
A) Lars is ridiculing Sims by taking away the kid's masculinity (like saying Sims is girlish) or
B) Lars' accent prevents him from pronouncing "Mister" properly