Immediately after Egon shines the spotlights at the villain, he is shown on the rooftop with Peter and Winston (instead of Ray, who's disappeared) when he should still be down at street level.
After Egon has connected his proton pack to the three spotlights, he is shown on the rooftop with Peter and Winston, instead of Ray, who's disappeared. The next shot shows the right combination of Ray, Peter and Winston.
When Egon leaves the other Ghostbusters and Winston yells "Wait, what shall we do?" Peter is replaced by a duplicate of Egon.
When Winston is introduced catching a ghost, he shouts, "Oh no you don't!" in Ray's voice.
The characters say the name "Samhain" as it would be pronounced according to English rules; the proper pronunciation is closer to "sah-win". "Samhain" is an old Gaelic word, which adopted the Roman script (also used in English and most other Western European languages) but with different pronunciation rules to match the language.