1960s-era assembly languages used self-modifying code to implement function calls without a stack.
Knuth, v1, 1ed p.182:
MAX100 STJ EXIT ;Subroutine linkage
ENT3 100 ;M1. Initialize
JMP 2F
1H CMPA X,3 ;M3. Compare
JGE *+3
2H ENT2 0,3 ;M4. Change m
LDA X,3 ;(New maximum found)
DEC3 1 ;M5. Decrease k
J3P 1B ;M2. All tested?
EXIT JMP * ;Return to main program
In a larger program containing this coding as a subroutine, the single instruction "JMP MAX100" would cause register A to be set to the current maximum value of locations X + 1 through X + 100, and the position of the maximum would appear in rI2. Subroutine linkage in this case is achieved by the instructions "MAX100 STJ EXIT" and, later, "EXIT JMP *". Because of the way the J-register operates, the exit instruction will then jump to the location following the place where the original reference to MAX100 was made.
Edit: It may be hard to see what's going on, even with the brief explanation here. In the line MAX100 STJ EXIT, MAX100 is a label for the instruction (and thus for the procedure as a whole), STJ means STORE the jump register (where we just came from), EXIT means the memory location labeled 'EXIT' is the target of the STORE. EXIT, we see later is the label for the last instruction. So it's overwriting code! But, many instructions (including STJ here) implicitly overwrite only the operand portion of the instruction word. So the JMP remains untouched, and the * is a dummy token, since there's really nothing meaningful to put there, it'd only get overwritten.
Self-modifying code is also used where register-indirect addressing is not available, and yet the address you need is sitting right there in the register. PDP-1 LISP:
dap .+1 ;deposit address part of accumulator in (IP+1)
lac xy ;load accumulator with (ADDRESS) [xy is a dummy symbol, just like * above]
These two instructions perform ACC := (ACC) by modifying the operand of the load instruction.
Modifications like these are relatively safe, and on antique architectures, they are necessary.
