A language's specification can forbid you from doing things that you probably didn't intend to do. Rust is an entire language intended to be safer by design (as opposed to C, which is the cause of a lot of secure issues). You can write less safe code in Rust, but you have to tell the compiler that you're doing it on purpose.

It's also much, much easier to introduce bugs in certain languages because of the way they handle errors, or because the syntax is confusing and ambiguous. Even permissive handling of boolean logic, like what you get in PHP/JavaScript ("0" == true, for example) can result in massive security holes.