The description for caltrops says "The first creature that moves into that square must..." before describing the saves that caltrops require the and the possible damage they can inflict. The description goes on to say "Once a creature takes damage from caltrops, enough are ruined that other creatures moving into the square are safe. Deployed caltrops can be salvaged and reused if no creatures took damage from them."
This is different from the description for marbles, which explicitly says that any creature entering their space scatters them and makes the space safe for later creatures.
So are caltrops capable of forcing a second creature to make a save against damage after a first creature has saved? Or are they consumed in the act of forcing a save, before a second creature enters their square?