If you load a C64 game, especially one on tape, there is a good chance for the data transfer to take a good while. Now imagine that all you see during that time is a blank static screen, and after 20 minutes you realize that the loading process froze 15 minutes ago, but the tape kept on running.
To let you know that loading is still in progress, many games display a graphical effect like this one:
 |
| This is comparatively subtle... |