Continuity mistake: When Lester is in Janey's room and he is looking through her phone numbers, when he gets to Angela's number it starts with 555 but when he dials it on the phone the first two numbers are a similar beep but the 3rd is a different tone. It looks as if he is hitting 553. (00:22:30)
Continuity mistake: Near the end of the film, Angela comments on Lester's shape. At this point, Lester is sitting on the counter with his arms stretched, supporting himself. But when Angela touches his arm, his elbow is bent. (01:32:57)
Continuity mistake: When Carolyn and Buddy are together at Mr. Smiley's drive-thru, Lester hears them at the order menu ordering, but when it cuts to Carolyn and Buddy in the car they are seen driving up to the order menu.
Continuity mistake: In the beginning of the movie, after "mentalboy" has filmed Lester and Janey in the kitchen, Lester looks out the window. The kitchen is lit up, and it is very dark outside. This would naturally produce a reflection in the window, making looking outside impossible. There is no reflection. (00:10:00)
Continuity mistake: In the first overhead shot of Lester in bed, just before his shower, his slippers are positioned in a V-shape, but in the next shot, from under the bed, has them parallel to the bed. (00:01:34)
Continuity mistake: When Lester is driving his Toyota after he quit his job, the plates on the car are different between the first shot (361813) and the second shot when he pulls in to the burger place (315502). (01:05:30)
Continuity mistake: When Carolyn calls Lester and Jane out whilst honking the horn, the shot changes from her to the house several times. Each time the shot is facing the house the side mirror is flapped in and while facing her it isn't. Additionally, in the same scene when Lester comes out, his briefcase opens and his cellphone falls out toward the car out of reach, when he picks it up again it's right next to him.
Answer: There's no mention that he had a mental disorder. He likely suffers from the emotional abuse his father inflicted on him throughout his entire childhood, which included sending Ricky to a military academy and forcibly committing him to a psychiatric facility (for unknown reasons). This probably contributed to Ricky's later eccentric behavior and drug use.
raywest ★