Take off/Land stream of perfectly operational ArDrone can be found in the "downloads" section;

Here are some examples (Y scale is the same):

1) Good working ArDrone. Altimeter shows~500-600, motors ar about the same and after initial bump stabilize at around 160.

2) Unstable flight caused by vibration of slightly bended axis. Note strange altimeter reading, amplitude of motor's variations and close-to-limit (255) values

3) Broken central cross. The crack may be even invisible, but "'softness" of one of the leg together with slight misalignment leads to erratic behavior.

Interesting, in examples 2) and 3) altimeter shows something wrong in spite of the problem is a "mechanical" problem caused by motors and the cross. Note, the altimeter shows correct and stable values if the ArDrone's motors are off and the toy is held by hand. Most probably wrong reading of the altimeter is due to strong vibrations.