Cf. Tolstoy: all happy families are alike, but every unhappy family is unhappy in its own way.
What happens twice probably happens more than twice: are there other notable expressions of this idea?
(There's a well-known principle in software development that's pretty close, though I can't find a Famous Quotation of it right now: when you're choosing a name for a variable or function or whatever, avoid abbreviations: there's only one way to spell a word right, and lots of ways to spell it wrong. Though this is not always good advice.)
Biblical verse on the asymmetry of error: "Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it."
Another month, another rationality quotes thread. The rules are: