For any type A -> B operation, it's possible to fail - that is the whole point of parsing into type B, to catch when B's invariants would not be satisfied. The

For any type A -> B operation, it's possible to fail - that is the whole point of parsing into type B, to catch when B's invariants would not be satisfied. The bug could be as simple as neglecting to handle that failure scenario. Some languages make this less likely (Haskell, Rust) but most mainstream languages will happily let you introduce this bug.