I find that occasionally, when using the pen, a line is drawn by the pen, even though it is lifted high above the screen. So I have to erase these unwanted marks. At that time, on clicking on eraser or on other buttons, the program does not respond. The "save" button does seem to work, however. It starts behaving normally after a while. That is when I can erase the unwanted marks.
Is there something I am doing wrong?