Graph Editor, what is the problem doc?

 When it's like this? how are you supped to know what's wrong...

 Each key point on an animation curve has a Tangent setting that specifies the curvature leading into that point.

• The default Tangent setting (Spline) creates rounded curvature—useful for animating attributes that change smoothly over time. 
• The Linear setting creates angular curvature, useful for a bounce effect or any other abrupt change in attribute value. 
•  Other Tangent settings are available for different animation effects. For example, a Stepped setting lets you make an instantaneous leap in value, useful when you want to flash a light off and on.

While splined tangents work well most of the time, they can also have some potentially undesired side effects. The most common is that Maya will overshoot attribute values in order to make a smooth curve. In this example, the lowest key values represent the bouncing ball hitting the ground. The spline causes the ball to go through the ground - an effect most noticeable after the second keyframe.

A quick way to fix the curve overshooting in this case is to "flatten" the tangents where the ball lands. Click the button. The curve is still smooth but all tangents are forced to horizontally align.

Another problem is Gimbal Lock, most starting animators get this as the they move the rotation axis until a axis is "locked" on the top of another.  Flip blog talking about Gimbal lock and the Eulier Filter .

If you want to learn more the 11 Second Club Blog with Graph Editor Tutorial, part 1

And good Spline habits through Spline Hygiene at Splinophilia

Thanks to:
https://www.cs.washington.edu/education/courses/458/09au/content/html/exercises/graph_editor_exercise.html
http://download.autodesk.com/us/maya/2010help/index.html?url=Keyframes_and_the_Graph_Editor_Using_the_Graph_Editor.htm,topicNumber=d0e13139