NULL pointer
Pointer that doesn't refer to any object. Dereferencing crashes the program (segfault in C/C++, NullPointerException in Java). Tony Hoare called null his "billion dollar mistake".
Source of a massive share of runtime bugs. Modern languages eliminate it: Rust has no null, uses Option<T>; Swift has Optionals; Kotlin has nullable types with ?; TypeScript strict-mode requires explicit null handling. Java got Optional in 8 (2014), but null remains. C++ has std::optional + nullptr. Static analysis (Infer, NullAway) finds null bugs in existing code.