The code can be unsafe because it is physically impossible to test for every input in a computer. This is where various engineering designs come in which reduce the area of testing based on some theories.

Also, even in math, there are enough mistakes in publications (not just typos, but reasoning errors) which hopefully do not affect the eventual results in any fundamental way. The equivalent of safe code in computer science would be equivalent of completely formal proofs in mathematics (like in Coq and similar languages), but probably much more difficult due to existence of temporal conditions.