levis1 That's/that was the only accident that's happened in my life. "That's" ( That is ) would be more consistent with the rest of the sentence, where you have "that's happened" (t hat has happened ) because 'is' and 'has' are the same tense. Still, it would not be a serious mistake if you used 'was'.
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levis1That's/that was the only accident that's happened in my life."That's" (That is) would be more consistent with the rest of the sentence, where you have "that's happened" (that has happened) because 'is' and 'has' are the same tense.