"a formula written on paper in pencil was improperly transcribed", "neglect to properly "seed" the program's random number generator, A HW bug that's not close to obvious numbers to check, intentionally inserted bugs, input outside of the intended design, etc.
offtopic, but a unit type would have prevented that. i had no idea how many errors i was making in my math programs before i started using F#'s type checker to make sure all the types lined up properly.
all of those can get caught with type checking, but it isn't perfect
With all the emphasis on testing and TDD, etc, I get the feeling that reviews are getting the shaft. They are both important, for different reasons.