private ip addresses from ISP

Hi all Have you had this experience? Our router is running BGP and connecting to our upstream provider with /30 network. Our log reveals that there are private IP addresses reaching our router's interface that is facing our upstream ISP. How could this be possible? Should upstream ISP be blocking private IP address according to standard configuration? Could the packet be stripped and IP be converted somehow during the transition? It happens in many Tier-1 ISP though ! Thank you for your information