As a network engineer/support tech I've had my fair share of problems with both the users and our own customer support desk. Eventually I learned to ask the same questions help desk should have already asked before heading to the customer's location...
One time I drove to the other side of the military base only to find that the user had two NICs and the connection was restored by plugging the lan patch to the other NIC. Help desk should have caught that...
Another time near the end of the day I was assigned a two jobs, one where an entire building lost connectivity and another where a single user had lost connectivity. Of course the building took precedence and by the time I was done with it I went home for the day. When I came in the next day the workgroup manager (the guy that does the troubleshooting on the customer's computer before calling in issues to the help desk) was being a jerk and he was acting like his user was giving him hell or something because I didn't show up the previous day... Turns out the
workgroup manager tried pluging a crossover cable to the user's workstation.

The workgroup manager was humble after I showed him the problem...
At my last base I wasn't doing network support and actually ended up being a workgroup manager myself. I went down to the base network control center to get a copy of the Windows image burned to disk so I could load a few workstations in my building... Well, the help desk guy that was assisting me decides to burn the image straight from the network share and I warned him he would coaster the disk if he lost network connectivity. About 70% through the burn, another help desk tech enters the office and discreetly unplugs this guy's NIC as a practical joke.... -_- Seriously wtf. I probably wouldn't have cared so much if they didn't make me bring my own disk in. For some reason they didn't have any DVD-Rs...