I think two very good (Perl) examples of this would be the usage of strict and the usage of symbolic references.

I think most people would agree that you should almost always use strict;, and you should almost never use symbolic references.

Many people would also argue that there are times when breaking those two rules may be appropriate. However, the general consensus is usually that you should never use a symbolic reference unless you can first explain why you shouldn't do it. And similarly with strict - if you can first explain why it is so important, then you probably know when it is safe not to use it.

So, I agree completely with Herkum. Learn the rules first, and understand them. And then go ahead and break them :)

Cheers,
Darren