var x = 1;
var y = "" + ++x;
This yields an invalid increment operand. Instead, the space between the "+" must be preserved. I also suspect (but have not tested) this may be a problem with this code too:
var a = 1;
var b = a++ + "";
I suspect the increment and the decrement tokens in the obfuscater just need to be tagged as space sensitive (both before and after)--so we don't remove the spaces when compacting the code.