Doctrine usually answers the question "why?"  principles usually answer the question "what?" Whenever we emphasize how to do something without reference to why we do it or what we do, we risk looking beyond the mark.  At the very least, we fall into the trap Paul described to the Corinthians, "For the lettter killeth, but the spirit giveth life" (2 Cor. 3:6).

Those who are committed to following rules without reference to doctrine and principle are particularly susceptible to looking beyond the mark.  Equally dangerous are those who get mired in rules and are thus less willing to accept change resulting from continuous revelation. 

"The ordinary reader must undoubtedly struggle trying to figure out what it means to 'strain at a gnat.' One might even guess that it means to strain one's eyes while looking at a gnat. The problem here, though, is not the word strain, but the little word at. This is a printing error that has persisted since the original 1611 publication of the King James Version. The translators intended this passage to read as follows: 'Ye blind guides, which strain out a gnat, and swallow a camel.' The Greek word here is diylizo, which means 'to filter out'. Figuratively speaking, the scribes and Pharisees could never tolerate a little gnat in their (or anybody else's) drink, but a camel could be swallowed whole. Jesus, of course, is referring to the strictness with which these legalistic Jews had interpreted the law, yet their concern for detail did not prevent them from violating the most important commandments in the law."