Plot hole: Near the end, Sam locks Mr. Howard and Ms. Briggs in a closet. Sam and Gibby then put a heavy desk right against the door and leave them stuck inside. Then later the two teachers are seen frantically controlling the students. How did they escape? They were inside a closet with the door locked on the outside and with a heavy desk against it.
Suggested correction: It's clear that another teacher or teachers heard the announcement or heard them yelling and let them out... What we should be asking is why no other teachers helped them control to the students.
Whatever Happened to Sarah Jane? (1) - S1-E8
Plot hole: When Maria Jackson is running away from the alien creature, she turns left into a metal fencing. The camera shows this seconds before she runs into it, banging on it in frustration. This does not make any sense. She saw the metal fencing, so why did she continue to run into it? Why did she not turn right instead and along the park?
Plot hole: When Justin and Juliet were trapped behind the glass and Juliet let herself be captured by the mummy, so she doesn't burn to ashes when the morning comes, why didn't Juliet just escape? The mummy gave her enough time to turn around and share a moment with Justin, she could've used that time to fight him off, break Justin out and escape. The mummy wasn't very strong and Alex would've shown up 20 seconds later anyway, there wasn't any real reason for Juliet to just let herself be taken away.