When to promp?

In a play I saw recently, an actor was prompted whilst in full flow. Both actors were happy with the dialogue and the audience, I’m sure, unaware of any error.

The prompt given, was a completely different line to that being spoken. The actors ignored it and carried on regardless.  It was obvious that the prompter was trying to get verbatim that which was on the page.

When does one prompt? Only when a deadly silence hits the stage? Or if the actor goes unnoticeably off track?