If you have unusual characters, like & in your browser title, or description or social tags, Rapidsearch Live will fall over. It took me most of the day and some additional help from a smarter person (thank you @instacks) to work this one out. So hopefully by putting it here it might save someone else the headache. And/or Will support requests for things that are outside his control (stupid users!). 
It may be that it’s intrinsically wrong to use & in these places, although I can’t say I’m aware of such, but nonetheless, if using RSL… Don’t do it!
I’ll be adding this to the Template Repo Knowledge base at some point.