The golden rule should always be " diagnose" then take the appropriate actions but not outside of the manufacturers QRH, which is the most condensed format for addressing non-normals. A " real life" example of crew messing up was last year when on a 738 one VSV sensor failed and allowed the engine to go to ground idle condition at 38000. The result was an " ENG FAIL" amber and the crew actioned the " severe damage" C/L. In fact time spent on diagnosis would have shown N1 and N2, FF, EGT and Oil pressure, a simple advance of the T/L would have removed the warning. The problem in the sim is limited time and crews tend to react in the way they perceive the instructor wishes, on the other hand instructors can often generate " out of the box" scenarios to test the crew. There is a thin line between negative training and practicing situations.