Thailand takes their king pretty seriously, and it’s against the law to insult him. Since the king’s picture is on the Thai currency, and the foot is considered particularly dirty, stepping on a piece of currently is equated to a direct insult of the king. This action could land you a beating, or jail.
Officers of the Bangkok Metropolitan Authority (BMA) frequently enforce this aspect of the public littering law. The fine is up to 2000 Baht.
Rumor has it that the penalty ranges from a slap on the wrist to a ticket for a few hundred baht. Thankfully the exchange rate is favorable and this is around $10 US.