Queries that reliably return eight results, suddenly return 800K, and next time return eight results again. Like a ghost it comes and goes, no one believes you until they somehow see it for them selves. Then it's gone like a leaf on a breeze. Non deterministic type coercion from mixed type arithmetic expressions, sometimes the result type is a floating point sometimes it is an integer. Why is it so, only the data knows I suppose. You too can experience such joys and wonders when working with relational databases.
Another reason to program in a very paranoid style, never trust any system or process other than the one you're writing. On second thought, don't even trust that.