Continuity mistake: During the scene in Mrs. Garrett's bedroom the flowers in the vase keep changing position.
Cousin Geri - S2-E5
Continuity mistake: Blair is talking on the phone to her mom. Tootie is listening pretending to wash the wall - her hand is raised. Blair says "Will you knock it off." The shot changes and Tootie's hand is now by her side.
Answer: The dramatic reason is to move the plot along. But in real life, store workers can be suspicious of groups of teens because of the perceived higher risk of Shoplifting. So the security guard may have seen them go into the changing room and decided to see what they were up to.