Although only has a six bullets only section on OWASP SCP Quick Reference Guide, bad practices of Output Encoding are pretty prevalent on Web Application development, thus leading to the Top 1 vulnerability: Injection.
As complex and rich as Web Applications become, the more data sources they have: users, databases, thirty party services, etc. At some point in time collected data is outputted to some media (eg. web browser) which has a specific context. This is exactly when injections happen if you do not have a strong Output Encoding policy.
Certainly you have already heard about all the security issues we will approach in this section, but do you really know how do they happen and/or how to avoid them?