First of all, you never fail these tests. Often time the word "fail" is used by police officers to indicate whether they believe you are intoxicated or not, based upon your performance on an individual Field Sobriety Test (FST). What the officer means, is whether or not you have "clues" that may indicate a certain level of intoxication.
For the Walk-And-Turn test, the officer looks for eight clues: (1) balance during instruction phase, (2) starting too soon, (3) stopping once started, (4) not touching heel-to-toe while walking, (5) stepping off of the line (or imaginary line - i.e., stumbling to the side, (6) using your arms to keep balanced, (7) turning incorrectly after step 9, and (8) taking the wrong number of steps.
It is suggested in the National Highway Traffic Safety Administration (NHTSA) manual that officers are train in accordance with that exibition of two or more clues during this test may indicate a BAC level above a 0.10 level. There is much debate on the accuracy of this, in as much, the NHTSA manual admits that if done correctly, it only has a prediction accuracy of merely 68%. This does not take into account a disability that one may be suffering from at the time of testing, or conditions, such as weather, etc. that may affect one's ability to perform such a test. Once those factors are accounted for, it many times comes down to a 50/50 coin toss, which is not statistically very convincing. When a test in no more accurate that a blind guess, it's not much use as an accurate indicator of intoxication. By itself, it's basically as valuable as the "magic 8 ball" in predicting whether someone is intoxicated.