On the Matter of Tense Shifts

Yesterday I sat in on a works-in-progress critique group. One writer's manuscript was interesting, had some nice description and a creative premise, but the writer shifted from past to present tense several times and changed points of view at least once.

While the POV shift was a minor distraction, I found the tense shifts confusing. I couldn't tell if the action was occurring today or yesterday or in the future.

While this isn't an excerpt from the work-in-progress, it is a crude example of what I mean:

"What's going on," he said.

"Not much," she answered.

"Do you think we'll get out of here alive?" John says.

"I hope so," says Ann.


**

The first person to comment picked up on the tense shifts and pointed out where they occurred. Before she could finish with her remarks, another person chimed in that wasn't quite right because the second person had read a book explaining it's okay to mix tenses when the writer is summarizing the scene.

I spoke up in favor of using one tense in a scene and sticking with it to avoid confusing the reader. I believe that whichever tense a writer uses, she should be consistent. 

Although I didn't get time to elaborate or give exceptions, such as if a writer adds in backstory or has a flashback, now I'm wondering if I mislead the other writers with my "be consistent" advice.

Okay, you expert writers. What are your opinions on tense shifts?