Many have probably heard about cross-site scripting or SQL injection and other common pitfalls when developing applications. There is a nice list of the top 25 most dangerous errors one can "create" during coding.
The list was compiled by many organizations and companies, and contains not just the errors themselves but also prevention and mitigations guidelines. Interesting read for anyone concerned about the security of their code.
2010 CWE/SANS Top 25 Most Dangerous Programming Errors