Most cryptographic operations are easy to implement. The best part is when you make a mistake, you'll most likely get random nonsense out the end. Floating point code is much more difficult, often you'll get an answer that's almost correct or worse one that's correct most places...
Heard about a great floating point implementation bug just today. When nintendo released Super Mario 64 for the Wii virtual console, they essentially just made their own emulator that plays a single ROM. The emulator is pretty accurate, though they made a mistake when implementing floating point operation rounding: All operations end up rounding upwards to the next floating point number (as opposed to alternating, depending on the result). A consequence of this was that some of the moving platforms in the game that had a periodic up/down pattern would slowly (over the course of literal days) creep upwards. Ended up being exploited in a speedrun category where the player attempts to press the A button as little as possible.
