It looks like the λJS has an issue parsing some of the escape sequences in underscore.js; it's a bug in WebBits parser that version uses.

Another implementation of the core semantics + desugaring approach, dubbed S5 (https://github.com/brownplt/lambdas5), is better engineered and actually uses SpiderMonkey's parser to handle more programs.

S5 runs underscore.js without any issues. Do give it a shot if you're still interested in trying things out.

Also, it's worth mentioning that S5 handles features of the new ECMAScript 5 specification, like strict mode, getters and setters, and new APIs, that the original λJS simply doesn't handle, so it's a much more complete account of JavaScript.