I've seen XML parsers that will convert element content from strings to native types by default. So "0" becomes an int, "true" becomes a boolean, and "null" becomes an actual null. I had to take extra steps to keep everything as a string unless explicitly told not to.
JSON does not have this problem, BTW.
There's probably a few things you can do live on a call.
I always wanted to try passing a common function through a minifier then a beautifier. Show them the code with unhelpful variable names, and ask them what name they would give the function.
A good programmer would be able to identify a string compare function or an IP bitmask eval function pretty quickly.