The Go `defer` keyword schedules function calls to execute before the surrounding function returns, but its behavior depends critically on return value semantics. When using unnamed returns like `return x`, Go immediately copies the value to a register, making it immune to later defer modifications. Conversely, named returns like `func f() (result int)` maintain a mutable variable in the stack frame that deferred closures can modify until the function actually exits.

This distinction explains why deferred closures capture variables by reference, supporting powerful patterns for resource cleanup and error handling. Internally, Go implements defer as a singly-linked list attached to each stack frame, enabling LIFO execution and panic handling. While modern Go has optimized defer overhead, developers should avoid it in extremely tight loops where millions of defer records could impact performance.