Home Alone (1990)
Heather's count goes like this: Fuller(1) Tracy(2) Heather(3) Jeff(4) Sondra(5) Linnie(Hand on Linnie's head, 'Buzz don't be a moron!') Buzz(6) Brooke(7) Megan(8) Murphy kid(9) Rod(10) Heather again(11).
When Kate asks her for the final count Heather says 11 kids including me, 5 boys, 6 girls which is correct but she just doesn't realize she counted herself twice. One theory is that Devin Ratray actually mistakenly put Kristin Minter off and the actress herself knowing the script called for 11 people assumed she hadn't actually counted herself and did so again.