Error causes pet to become pest instead
I believe the answer is:
sin
'error' is the definition.
(a sin is a moral error)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sin that I've seen before include "grave offence" , "Breach of moral law" , "Breach of moral code" , "Transgress" , "Breach of morality" .)