Its something that you always want to skip for the sake of time, but working methodically, layer by layer of the OSI model saves you from huge, time consuming issues down the road.  More specifically, take your time to test new WAN or LAN connections.  The tests don’t have to be complex,  something like ping testing over a period of time is far better then assuming the connection is good and doing nothing.  Its important to catch errors like this before you start adding on to the base layer from a customer perspective:  Once you have everything installed, and there are problems with the physical or network layer, from a customer perspective  your setup doesn’t work right, not that the connection is bad.  Its much easier to explain a delay in implementation due to issues you found when testing prior to your install. 

Just one of the simple things that I need to keep reminding myself of.