Further to the previously mentioned 40 minute difference on the clock, the episode "Earshot" is riddled with inconsistencies in the time-line.
The following is shown as chronological, but watch the clocks...
11:00 Jonathan starts putting gun together.
Moments later, the Scooby gang are in the newspaper office quizzing Freddy and the clock behind Oz' shoulder reads 8:40
Discovering Jonathan's letter, Xander runs into cafeteria looking for him, and the clock there reads 10:35
The next thing we see is Buffy in the courtyard, where she spots Jonathan in the clock tower (still putting that gun together!) It's now 12:40 (so far it's taken Jonathan an hour and 40 minutes to put that gun together!)
Buffy runs 10 yards and checks to see Jonathan still in the clock tower, but now it's 12:10! (Boy she's a fast runner!)
After disarming Jonathan, Buffy heads to the cafeteria to fight with lunch-lady Doris, where it's suddenly 12:40 again!
Maybe Sunnydale high should get a technician in the check out their clocks, they definitely need new batteries!