As a kid, I always thought the slime made the toast. As I got older though, I just assumed Egon poured slime over bread that happened to already be in there and that the experiment was just about making the toaster dance.
Anyway that aside, my ultimate GB2 mystery is this deal with Ecto 1A’s license plate. In almost every shot when they’re at the museum in the final battle, the license plate has the Statue of Liberty to the immediate left with ECTO 1A stamped to the right of it. But then during the close up, it has the SoL in between “Ecto” and “1A”. And don’t even get me started on the random Ecto-2 license plate sighting just before they go into the sewers lol.
Aesthetically to me, the close up version looks best. I just don’t get why there was so much back and forth on which version to use and how they let so many different ones end up in the final edit.