Will flip upside-down when Isaac tries to collect it. Afterwards, it will get stuck on the ground, unable to be picked up unless blasted off the ground with an explosion. Bumbo and Bum Friend are unable to pick this up; they instead hover over them attempting to pick them up.
Unlocking Sticky Nickels is strictly speaking a downgrade as they replace normal nickels, though the sticky ones are rare enough to not make a severe difference.
Although Edmund McMillen has stated they should replace pennies, this is not how they are actually implemented.