> Did this choice of a small speed boost over compatibility ever haunt the decision makers,

...in the end it's just another calling convention which you annotate your system header functions with. AmigaOS had a vastly different (very assembly friendly) calling convention for OS functions which exclusively(?) used CPU registers to pass arguments. C compilers simply had to deal with it.

> What could possibly go wrong?

...totally makes sense though when the caller passes arguments on the stack?

E.g. you probably have something like this in the caller:

    push arg3      => place arg 3 on stack
    push arg2      => place arg 2 on stack
    push arg1      => place arg 1 on stack
    call function  => places return address on stack

...if the called function would clean up the stack it would also delete the return address needed by the return instruction (which pops the return address from the top of the stack and jumps to it).

(ok, x86 has the special `ret imm16` instruction which adjusts the stack pointer after popping the return address, but I guess not all CPUs could do that back then)