Penny for Your Thoughts - S1-E15
Revealing mistake: In the first scene the clock shows an impossible configuration. The hour hand is slightly before the five, and the minute hand is between one and two. If it's about seven minutes after the hour, the hour hand should be slightly past one of the other numbers, not slightly before it. (00:00:24)
Suggested correction: Not sure if this is technically a mistake or not. These clocks run on batteries, and when the battery's time is almost up (no pun intended), the weight of the hands sometimes over-rides the battery's strength to move them properly. This is likely a dying battery that failed to move the hour hand correctly. So while it is 'a mistake', it's part of real life, and not really a 'mistake in the episode'.