Starring: Nathan Fillion, Richard T. Jones, Alyssa Diaz, Titus Makin Jr.
Casualties - S2-E14
Plot hole: When Bradford and Chen arrive at her apartment to see the damage her newly adopted dog did, she discovers the dog ate her roommate's varsity jacket, but she had to turn the doorknob to open the door; there is no way the dog got into that room if the door was closed.
Revelations - S3-E6
Continuity mistake: When Nolan is asking for advice from his ethics teacher, she says, "It's 2021." However, with the 2019 start time for the rookies and a supposed year's probation, this shouldn't be possible from the show's timeline perspective.
Suggested correction: Nolan was not a rookie when he joined the ethics class; he was already a P2 and finishing his college degree to become a training officer.
The previously suggested correction is incorrect; he is finishing his college degree to become a training officer, but he and his fellow rookies are not at the P2 stage for roughly another month (with Nolan having an additional month after that after the events of Season 2). The original continuity mistake at the top of this thread is 100% accurate.
The Choice - S5-E4
Trivia: We see Bailey and her team of firefighters arrive at the house on Engine 118, which is the same engine that Captain Nash is in charge of on 9-1-1, also set in Los Angeles.