、。
When I made a mistake, he defended me.
Literal
I [subject-が] mistake [object-を] committed time, he [topic-は] defend-[for-me-くれた].
過失 ('mistake, error [caused by negligence]') is a formal/legal-flavored word — different from the everyday 間違い. 犯す ('to commit [an offense]') is the formal 'commit' verb for mistakes, crimes, taboos. 弁護する ('to defend [legally or verbally]') is similarly formal. The ~てくれた marks the kindness done to the speaker.