This arises when the consequent ("the room would be dark") has other possible antecedents (for example, "the lamp is in working order, but is switched off" or "there is no lamp in the room").
ðɪs ɚɹˈaɪzᵻz wɛn ðə kˈɑːnsɪkwənt"ðə ɹˈuːm wʊd biː dˈɑːɹk"hɐz ˈʌðɚ pˈɑːsᵻbəl ˌæntɪsˈiːdənts fɔːɹ ɛɡzˈæmpəl, "ðə lˈæmp ɪz ɪn wˈɜːkɪŋ ˈɔːɹdɚ, bˌʌt ɪz swˈɪtʃt ˈɔf" ɔːɹ "ðɛɹ ɪz nˈoʊ lˈæmp ɪnðə ɹˈuːm".