The arctil challenges provide a number of code examples which you can take a look at, each will offer different examples of how applications can be vulnerable. Your task, find each vulnerability.

PHP Loose Login
Python What a Pickle
Javascript Search
PHP File Upload
PHP Dynamic Articles
Ruby Search
Python API Trouble
Python Paste Reader
PHP Password Reset
Python User Registration

Feel free to brag about all the vulnerabilities you find, you can tag us on Twitter.