Testing a packet sequence number checker. The packet sequence number checker may check a packet-based data communication between two interconnected devices. An error injector may be provided in-between the interconnected devices such that the data packets may be received from one of the two interconnected devices and may be sent to the other one of the two interconnected devices by the error injector. A received packet is randomly selected from a packet data stream between the two interconnected devices and stored in a buffer. A length of a later received data packet from the same sender of the two interconnected devices is compared with the selected buffered data packet, and the later received data packet is replaced by the selected buffered data packet.