To reproduce ProIcon's bug:
1: Draw a few random shapes
2: Select the filled-circle tool
3: Click the center of the canvas and hold
4: While holding the mouse down make the circle huge then small again
5: Notice that when shrinking the circle-size, what would have been under it is erased
Now on to my suggestions:
JSlint your code: I know it's a hassle, but it really does make reading code easier.
You can use '' instead of having to escape double quotes. Another way to make things easier to read