Best practices are rarely the best — they're mostly just cargo cult common practices. And as more people adopt them, the more mediocre they become. The best is usually what most people aren't willing to do.
Following conventional wisdom and relying on shortcuts can be worse than knowing nothing at all.
Ben Horowitz • The Hard Thing About Hard Things: Building a Business When There Are No Easy Answers
Unless you’ve actually done the work, you’re in no position to encode it as a best practice.
Jason Fried • It Doesn't Have to Be Crazy at Work
Judging by the similarities in the layouts and design within industries, most websites are based on copying competitors. This follow-the-average approach leads to poorly performing elements becoming accepted as “best practices” just because they’re common.
Chris Goward • You Should Test That: Conversion Optimization for More Leads, Sales and Profit or The Art and Science of Optimized Marketing
Probably the biggest lie told in schools, though, is that the way to succeed is through following "the rules." In fact most such rules are just hacks to manage large groups efficiently.