Thanks! The only remark (sorry, as my usual :) I was just looking at the code related to this issue myself and noticed that for it to be fixed (in easy way) the error trigger at https://github.com/less/less.js/blob/master/lib/less/tree/mixin-definition.js#L99 has to be eliminated somehow...
Now since this change actually makes that error code unreachable (or at least I can't invent an example where it could now) - would not we want to eliminate that never used code too?

On the other hand it never harms to keep it "just in case" (so I'm actually only concerning of it to be misleading for further changes). Any ideas?

@seven-phases-max I sometimes put equivalent of "Bug: unreachable code reached please open new issue" into code. Theoretically, it could help with early error detection. Practically, I seen such report in tracker maybe once, but it helped find errors when refactoring a few times. It also helped to remember that branch should not be reachable few months later :)