Analog Switches (AS) have played an essential role in a large number of Mixed Signal (M-S) circuits. Depending on the use of the AS, designers have optimised their topology to the needs of each specific switching function. The success of field programmable devices in the digital domain has motivated some manufacturers to explore similar solutions to fast prototyping in the analog and M-S domains. In this work we explore the defective behaviours of programmable AS under realistic catastrophic and parametric defects. A classification of the defective behaviours for bridge and open defects is done. It shows that the simple fault model, with faulty state of permanently transistor stuck-on or stuck-off, is not sufficient to reflect the real behaviour of the switch.