Sometimes an error slips through during production. Collector's often like these for the rarity.
Sometimes a bit of metal gets mixed in the glass and ends up on a marble
Definition of bifurcated : divided into two branches or parts
Pieces of oven brick sometimes find their way into a marble
Cullet are leftover pieces of glass from a marble run. They can be fun to collect.
Double stamped comic marble
Sometimes things got horribly wrong!