To err is human, to really foul up requires the root password.
More generally, it taught us to be careful with string substitution and interpolation.
Well, "us" except Unix and Unix-like developers, where "rm -rf /*" still works like it always did, unless your rm added --no-preserve-root or some similar ugly hack.
There's actually (at least) two versions of GP's comment - unsafe code/data, what you appear to be referring to:
rm -rf $FOO/*
And typos, which is what I first thought of upon reading it: