And Memento is arguably one long series of flashbacks, but done uniquely and well.
I only just saw that movie though I had heard about it before. Thanks for bringing it up Quantus.
In my Film Theory & Writing Classes
Momento was always cited as the textbook example and best explainer of the difference between Plot and Story and the difference between chronology and linearity.
Going back to the original question of the thread, I think context is important(like rereading the original question):
There is a flashback in my novel that isn't very long, just a few lines of dialogue and some action, at a moment when the hero needs to make an important decision, when the character can either evolve or devolve (so to speak). But I keep reading about how flashbacks are completely taboo, you should never do them, period. Is this true?
In this case, you have the tipping point for the character. When the character's Desire Line(The progression of character getting closing and closer to his/her goal) and the Need Line( The progression of how the character has to change to achieve his/her goal). This is actually a really good time to Flashback if you want to illustrate the thinking process of your character.
The key here songofsuzanna is how you present the source of the flashback. If the 3rd Person Omniscient Narrator all of a sudden went, "Back in time, some years ago..." it will break the linearity of the action and the flow is dead. That's a bad move.
If the flashback is an illustration of the character remembering that event and said flashback is the motivation they need to make that crucial decision, then the Flashback works and drives the story to its conclusion.
Basically I'm saying, make sure the Flashback is motivated by something going on in the scene not because the Narrator wants to. IF you have a first-person narrator, it makes it that much easier because it's someone telling the story in their own words.