Using an access point not suitable for a specific environment. What works well for a private home network, might fail in a large business environment.
Placing access points without any strategy. Perform a WiFi site survey with NetSpot to determine optimal placement and number of devices.
Not staying in the know. It is not just setting up the network that keeps it functioning well, it is also proper maintenance. Keep your business safe and unexposed to security risks, keep up with what's going on with the wireless market and where the technology is headed. Staying ahead of any possible risks will save time and money.
Not thinking ahead. Don't just think about your immediate needs when setting up a network, try to think how it will develop and what your business might need from it in the future.