Curves going past keyframes

Hi everyone,

I seem to have a problem with my animations. I'm making simple animations such as rotating cube or a moving platform directly in Unity. And the problem is that after i m've made my animation from a keyframe to another keyframe, sometimes when i move the timeline bar in red after the last keyframe, my curves continue after the last keyframes.

And when it's done i can't go back to my previous state. I can push the timeline and so the curves are longer as i move my timeline bar but I cannot "crop" the curves.

Here's a screeenshot of my problem

Screenshot