A split pair is a wiring error where two connections that are supposed to be connected using the two wires of a twisted pair

Twisted pair

Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...

 are instead connected using two wires from different pairs. Such wiring negates the benefit of using twisted pair in the first place and is likely to cause errors in high rate data lines and increase interference

Interference (communication)

In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...

 on voice lines, especially on long runs.

When using registered jack

Registered jack

A registered jack is a standardized physical network interface — both jack construction and wiring pattern — for connecting telecommunications or data equipment to a service provided by a local exchange carrier or long distance carrier. The standard designs for these connectors and their wiring...

 connections and other connection schemes that use modular connector

Modular connector

Modular connector is the name given to a family of electrical connectors originally used in telephone wiring and now used for many other purposes. Many applications that originally used a bulkier, more expensive connector have now migrated to modular connectors...

s it is very easy for the unwary to introduce split pairs. The easiest way to wire such a connector is to put the pairs next to each other but this will not result in correct pairing.