This issue occurs when one or more persons in you party is experiencing at least a 10% packet loss. Packet loss can cause latency or a delay while online. So if you are seeing a desync when in online co-op where one user is seeing something the other is not or if you are seeing slight mouse latency this is usually the cause.
You can try restarting your router to help relieve the latency and the loss of packets. Sometimes modems pick up interference from nearby networks. You may also wish to place your wireless router or computer in a different area since latency can be caused by conflicting signals.