Verb tense shifts
Sounds exciting, doesn't it? In a nutshell, it means that a story starting out in present tense suddenly, and for no reason, shifts into past tense (or, vice versa). Here's an example (not an actual student example, just a quick, made-up one):
Melanie walked to the counter and examined the envelope again. She notices something this time--the initials "PK," smudged in pencil at the corner. Pushing down the anxiety, she reached for the knife and slices open the envelope.
Look at all those verbs - past tense, then present, then past tense again, and so forth...
Because that very first verb ("walked") is in past tense, the rest of the entire story should be written in past tense. (And - if a story begins in present tense, the only excuse for shifting to past tense is a flashback).
My students always tell me, "I guess I do that (verb tense errors) because it's the way I talk." A very good point! I think that's exactly why these verb errors are so frequent. We're all guilty of shifting in and out of past/present when we speak, not even giving it a second thought.
But in a piece of writing, that sort of shifting is distracting for the reader. And, more importantly, it's a grammar error that should always be avoided.