You only pay for what you get.
What is refunded
An image, edit, tool or video that fails: all its credits come back.
A job that returns fewer images than you asked for: the credits for the missing images come back.
A voice-over that fails: its credits come back.
A podcast episode whose transcription fails: its credits come back. If only the show notes fail, trying again costs nothing.
Where to see it
Open Plan & credits and look at Credit history. Refunds show as a plus line, such as "Refund: generation failed".
Credits go back to the place they came from: plan credits to your plan, top-up credits to your top-ups.