Because with software you can’t generally predict the consequences, even more so as software tends to not be static, but evolves and starts to interact with more and more other software.

Validate your inputs. Be very careful with in-band special values and escaping syntax. Don’t make any assumptions about what is “reasonable” input. If you have to make assumptions, document them and validate all input for conformance. Always check what requirements and preconditions the code you call has on the values you pass to it. Don’t just make assumptions about it.