Possibility of referencing mixing to itself

[moseleyi]

Is it possible to reference mixin within itself, for example:

.rad(@r=5){
    border-radius:5px;
    &:hover{.rad(10);}
}

Seems that I get 500 error during compilation if I attempt to do it

[seven-phases-max]

It is possible. But you have to stop an infinite recursion somehow - for example see Recursive Mixins (in your snippet you simply have an infinite chain of :hover:hover:hover:hover:hover...).

For a theory also see Recursion.