I came across this article on the BBC yesterday, and it's really interesting to see just the sorts of error they are picking out. It's also really important that this information is spread as widely as possible, as everything that has been highlighted is relatively simple to fix. If two of the 25 can account for 1.5million website breaches last year, then I'd be interested to know how many the 25 as a whole can account for! The 25 villains include:

Improper Form Input Validation
Failure to maintain SQL Query structure (SQL Injection)
Cross-site Scripting
Hard coded Passwords

And a whole bunch of stuff which I'd have thought is screamingly obvious for any web designer who has half a care for avoiding these sorts of problems. A lot of the remaining 25 refer to server configuration, but there is something every single coder can do to help kill off this problem... Learn to code properly!