Motion Paths repositioning itself
Often, shortening a motion path to about half of its original size will cause it to reposition itself. To put it in simple, let's say if you are adding a Motion Path (Bringing up the Custom Animation, then click Add Effect > Motion Paths).
Next, you want to adjust the Motion Path by shortening it.
Unfortunately, everytime you try adjusting the Motion Path, it jumps to another location / position on the slide. Therefore, you will often need to move the Motion Path back to its original location / position manually. This step is definitely tedious and takes up quite an amount of time if you are intending to adjust the length of the Motion Paths frequently.
Here's a quick and easy solution which works for most of the time, by allowing the Motion Path to snap itself back to the original position:
Instead of moving the Motion Path back to its original position manually, move your mouse over either ends (The red or green arrow) of the Motion Path. After which, click on the ends once and it will snap itself back.