Once in a while, when you are editing your template, you'll get this error. The dreaded bX-btcv0r error.
It seems to appear only if you click on the Edit HTML and Expand Widget Template. If you did not click on Expand Widget Template, you'll NOT get an error.

But what's the point, that would mean you still cannot edit your templates.

There is a cure.
I found out the cure quite by accident, all you need to do, is to close all your browser and CLEAR YOUR CACHE. Then reopen your browser and the problem goes away. Google needs to look into this issue or else a lot of non technical people will just be stuck and give up.

Hope this helps!