Question

Which sentence contain an unnecessary shift in tense?

If anyone knows the answer, she does.

As I came in the front door, Eddie is going out the back door.

A row of ants had filed up the chair leg and was about to consume our lunch.

The barrier will drop down every time that a train approaches the crossing

Answer 1

I’d definitely say the second and third.

“As I came in the front door, Eddie is going out the back door”

The tense changes from past (“came”) to present (“is going”).
The sentence would make more sense if it said

“As I came in the front door, Eddie was leaving out the back door”

- in order to keep the tenses consistent.

“A row of ants had filed up the chair leg and was about to consume our lunch”

The tenses don’t actually change, but the grammar is just incorrect.
It would make more sense if it read as

“A row of ants had filed up the chair leg and were about to consume our lunch”

Hope this helped! :)