duplex mismatch is only a problem in situations where the end devices are contending for access in the same collision domain e.g in a hub. In these situations half duplex is used with CSMA/CD media access, which actively looks out for lan collisions before transmitting. Full duplex mode disables CSMA/CD therefore if a half duplex and a fullduplex device were plugged in to a hub, collisions would occur as the full duplex device is not listening for other devices before it transmits. Switches, however act as a multiport bridge in that each port defines its own collision domain and is totally separate to other devices plugged into a switch. The only problem here would be a duplex mismatch between the switch port and the end device plugged into it, generally in these situations you would use full duplex (if the device supports it) as this increases the bandwidth available to the end user.
