In the voice credits, the character Fabian Menkle is listed as "Fabian Men".
When Shaggy and Scobby burst in and crash into Velma, the hand holding the postcard doesn't have a glove, but the next scene does when they crash onto the floor.
Right after the snowman's first attack, the opening credits are shown and, surprisingly, within those 30 seconds, a snow blizzard occurred.