One of the aspects of Ethernet cabling is that there can be more to a cable problem than conductors being cut (open) or shorted. Brian Karas mentioned the integrity of the twists, which is critical to what frequencies the cable can handle well.
Most Ethernet switches auto negotiate the speed at which they connect. If a cable is stretched or kinked or the twists are untwisted too far back from the connector, the cable can't handle the frequencies of the higher data rates.
Thus, for example, in a 1 Gbps LAN a cable might have its connection negotiated down to 100 Mbps or even 10 Mbps and still pass low levels of traffic. In such a situation a small video stream may be displayed in a browser window or client application with no problem, but trying to display 8 cameras at once fails.
I have seen this kind of thing much more often than I have seen cuts or shorts in network cable. If the switch is out of sight, such as when you connect to a wall socket or you are out at the camera end, you won't see the switch's LED light that indicates the low speed connection.
I wrote a column about this not too long ago: Don’t Get Kinky, which among other things talks about cable testing.
I have also seen cables run over fluorescent lighting, near motors, and even strapped to AC power cables. Strong electrical interference can cause some data packets to be corrupted even if there is nothing wrong with the cable. When packets get corrupted after leaving the switch, the TCP protocol calls for retransmitting the packets that failed to arrive intact. A high level of retransmission can reduce the overall capacity of the connection, with the result that a low level of small data packets will get through, but in a high volume of large data packets some packets won't make it.
Good training in network cable installation, whether formal or on-the-job, is important.
I have seen jobs where poorly performing cable went undetected until the "second round" of cameras got installed, and then problems were seen.