Pointers
Defects in pointer arithmetic, where an address is adjusted by an offset whose derivation is unclear or unjustified. A magic increment, a hand-computed stride, or an offset that ignores element size moves the pointer somewhere the author cannot account for, and the resulting access reads or writes memory outside the intended object.
The theme is arithmetic on addresses that is not tied to the layout it is meant to traverse, which turns a small reasoning error into out-of-bounds behaviour rather than a wrong-but-contained value.