Missing cases, the "Oh, it does fail if the size of the input is divisible by 256" feeling. Or the O(n*this-exponent-is-way-too-large) algorithms that seem fin

Missing cases, the "Oh, it does fail if the size of the input is divisible by 256" feeling. Or the O(n*this-exponent-is-way-too-large) algorithms that seem fine with a ten item input but not so nice with 10,000 items. Or the incomprehensible build script that you (meaning me, of course) just kept adding things to with no systematic naming or sections. Just three that have bitten me in recent memory. Even the simplest and most routine tasks can get messed up sometimes.