My favorite: eat sensibly, exercise moderately, die anyway.
As a corollary, the injunction “live fast, die young and leave a good-looking corpse” in practice turns out to be “live fast, die in middle age and leave a prematurely old, ravaged corpse”.
Less macabre and more absurd, there was an SDMB post a while back about stubbing one’s toe hard and that it would eventually happen again, ending with “and there’s nothing you can do about it” or something like that. Still makes me laugh.