Crossed wires

"Crossed wires" can describe either of these situations: 1. when people who are supposedly having a conversation with each other are actually talking about completely different things 2. when a dialogue gets out of sync because people are not listening to each other - in a written dialogue this is usually because someone has posted twice before reading a reply. The expression derives from ye olden days of analogue telephones. Sometimes when speaking on the phone in the 60s or 70s it was possible to hear another conversation on the same line. You might also pick up the phone and find that instead of a dial tone you could hear a conversation. This situation was called "crossed wires".