Why does my school do the pattern differently? (And why should I believe your version?)

Patterns frequently vary from school to school for several reasons. One may be a lack of continuity in curriculum among different schools in the same organization. Another may be that traditional instructors don't want to incorporate newer patterns and/or styles of technique into their teaching. Perhaps a very liberal instructor has taken to altering the pattern himself in order for it to be more in line with his own style. Also, patterns naturally and frequently mutate at they are passed down from one master to his student instructor. Any or all of these things might contribute to any number of differences you find between the patterns you study under your Master and those that are contained in these pages.

"If mine are different, why should I believe your version?"
I have gone to great efforts to make sure that the patterns contained on my site are the most 'official' versions. For the WTF series, that means having compared what I learned under instruction from different WTF black belts with what I have researched through official WTF videos and books. The same goes for the ITF's Chon-Ji pattern set -- I have not simply taken my own instruction in the Chon-Ji patterns, but have checked the forms against several available text and video references. In addition, I have received email correspondence from both WTF and ITF practitioners verifying the movements and calling for changes when errors have been noted. I feel you can be absolutely certain that the WTF and ITF patterns contained in my Taekwondo Hyungs site are the official patterns taught by their respective governing bodies. The Kicho, Kuk-Mu, and Ki-bon patterns, as well as the traditional black belt forms Bal-Sek, Chul-Gi and Kong San Koon were more difficult to verify but, nonetheless, were checked against several different sources. The Shotokan karate patterns are the least authoritative patterns included. Even with these, though, I checked the patterns that I had learned against those in printed references before posting them to my site.

The most common discrepancies I encounter fall into two categories -- "height/level" and "open/closed".  The "height/level" differences are those where one school practices XXX pattern's punch at middle (chest) level and another does it at high (head) level.  (XXX is any pattern's name.)  It is a difference in height, or target, of the technique.  The "open/closed" differences are those where one school practices XXX pattern's block open-handed (i.e. with a knifehand) and another practices that same technique with closed fists.  How serious you believe these discrepancies to be depends upon you and your instructor.  Here are some example "coping skills" to deal with any discrepancies you find within your school or between your school and my patterns.
Very Liberal It doesn't really matter whether a certain technique is head or chest level, so long as it's done properly.
Liberal It doesn't matter whether a certain technique is head or chest level, so long as you are sure to do it the exact same way each time you perform the pattern. 
Moderate The pattern should be done exactly as your instructor teaches it, will little to no variance among students.
Conservative The pattern should be done exactly as your instructor teaches it with no variance.
Very Conservative  The pattern should be done exactly the way your instructor (or governing body, or original master, or creator of the patterns) intended for them to be done and this should be validated by other sources.

If your school teaches a pattern differently from how it is listed on this site, there should be no cause for alarm nor reason to refute your instructor. Your Master is passing on something very special to him, perhaps very unique to his style of teaching. It may even be something lets others know, "Ah, you studied under Master So-and-so." This is not something you should be worried about. There are several discrepancies in the way I was taught to perform the Chon-Ji pattern set. Nonetheless, my pages on this site reflect the researched and validated versions, and not simply what I was taught. If you study the patterns as outlined in these pages, you can be certain that you are performing them as they were meant to be handed down. If, on the other hand, your school practices them differently, enjoy and appreciate the differences. Ask questions. Most importantly, pursue Taekwondo to your fullest.