One of the difficulty switches is used for a pause feature on Kool Aide Man. When the switch is in the pause position on startup, the game freezes on the title screen. If you flip the switch, the game should start normally.
I just did - - no go! KOOL AID MAN on the native cartridge does not boot! Well, it does, but it just freezes on the opening title screen.
The difference that might appear on your console is the score display. On some consoles (like Thomas Chinese Jr.) the score digits overlap. This constantly triggers the collision detection which results in Kool Aide Man constantly bouncing back and forth in the top left corner of the screen. This makes the game unplayable.
Ah-ha! I will check again to see if the difficulty switch was flipped one way or another. That must be what it was - - I always thought the "freezing" in this game was the incompatibility bug! Good to hear that it's the score display, I'll check it and let you know.
On all (?) 2600 Jr. consoles (NTSC and PAL) that are made in China after late 1989 the signal timing in the TIA for the shifting pulses for the movable objects seems to be a little off from older versions of the TIA. Therefore when a game starts the shifting process in such a way that some of the shifting pulses happen during the visible part of the screen, the movable objects might be positioned slightly differently. This affects all PAL 7800 consoles too, BTW.
Hmm, noted! I'd be interested in knowing more about your Chinese PAL Jr.!
Something similar seems to be going on in your "unicorn" console too. You can see tis in your Pitfall II video. The game doesn't do an HMOVE in every line, so not all lines start with an eight pixel blank. Pitfall II tries to hide it with displaying a black eight pixel wide copy of missile 1 in those lines where it doesn't do an HMOVE. On your 2600 these missiles are shifted one pixel to the right. If you run Pitfall II in Stella and disable missile 1 with ALT-V, you can see that there are gaps in exactly the same scanlines where the one pixel shifts are in your video.
Other games worth testing for compatibility would be the starfield effect in Cosmic Ark, the tile screen in He-Man and the cobra head in G.I.Joe / Action Force.
Thank you, sir! That is all VERY helpful and informative!
I'll try those games and report back with compatibility.
Regarding the other side conversations on STELLA compatibility and TIA revisions, it's all very interesting, too.
It is strange to me that ATARI would be so short-sighted as to make revisions to the core of the ATARI 2600 system that broke the basic programming conventions that they initially had established!