Animation goes wrong when adding new Keyframe.

So ive had this problem for so long, that i finally decided to ask if im doing something wrong or is the Unity's Animation tool supposed to be just like this. Sometimes when i add new keyframe to my animation, that keyframe "destroys" the earlier part of animation. Here is nice example pic of the problem..

When i add the keyframe and this happens, im doomed to fix the animation AGAIN from very beginning since every keyframe before the new goes wrong. Its incredible annoying.

Am i doing something wrong? I really hope i am, since if this is how the Animation tool works my game is doomed since i dont know how to use any other animation tool / program. And use it fairly simple, i position bones, move the red line forward and position bones again.