nothing is certain but the unforeseen

/nˈʌθɪŋ ɪz sˈɜːtən bˌʌt ðɪ ʌnfɔːsˈiːn/

/nˈʌθɪŋ ɪz sˈɜːtən bˌʌt ðɪ ʌnfoːɹsˈiːn/

used to imply that life is uncertain and unpredictable, and that unexpected events or circumstances can always arise, making it impossible to be certain about the future