ZigBee and WiFi channels both exist in the 2.4 GHz band, existing in the exact same frequency space. When deploying both WiFi and ZigBee in the same environments, careful planning must be performed to make sure that they don't interfere with each other.
Operating a ZigBee network and a WiFi network on the same frequency will cause them to interfere with each other. Usually, the ZigBee network will take the hit.
ZigBee and WiFi Channels
ZigBee and WiFi channel numbers may seem similar, suggesting that they won't overlap. Unfortunately, this is not the case.
2.4 GHz WiFi Channels
2.4 GHz ZigBee Channels
WiFi's three non-overlapping channels (1, 6, and 11) use the exact same frequencies as ZigBee channels 11-22.
ZigBee channels 25-26 aren't immune either, because they can be caught in WiFi channel 11's sideband lobe (see Sideband Lobes below). ZigBee channel 26 is usually relatively unaffected by WiFi, but many ZigBee devices do not support it.
When a WiFi network is on the same channel as a ZigBee network, the WiFi network will usually interfere with the ZigBee network.