The Worst Day Since Yesterday - S3-E8
Other mistake: At the start of season 3 it is established by Whitey that the lockers are in alphbetical order, hence why Nathan, Haley and Lucas all have lockers next to each other, but in this episode when Haley and Rachel decorated the Scott brother's lockers, Brooke then goes to her locker and is sad when she opens it as Lucas decorated it earlier in the season. Brooke's locker however is only a few lockers away from Lucas', even though they are meant to be in alphabetical order.