So what do I mean by this. It means that you should feel free to modify a fly. For example, I like my duns to sit fairly low on the water so I often will trim the hackle on the bottom. Then they'll ride a bit lower.
Another example is the St. Vrain Caddis. The version we sell has a tail, most of those in the stores don't. I happen to like the tail, but if you don't just nip it off.
Sometimes making a small change can make all the difference in the world. The fish may have seen 10,000 patterns that look like X, but they may have not seen one that looks like Xa (slightly changed). This maybe just enough to fool them and/or to peek their interest.