Similar to MacGyver's issue, I had one PC in a hospital that would drop from the network intermittently. Took a while, but we found that the cable had been run directly on top of a fluorescent light ballast in the maintenance storage room below. When the guys went in and turned the light on, the PC would drop.
In another instance, a local optometrist built a new office with open floor plan, where he let the electrican pull the data and phone lines. You guessed it: "Sparky" pulled phone and data through the same conduit as the electric under the concrete slab. We can run the network dependably at only 10mb/half duplex.
On the other hand, we do some large-scale data centers where we have the patch panels on the same racks as the servers. I have data lines (5e) running in the same management and across the same ladders with both 110 v and 230 v power for the servers with absolutely no problems.
We are meticulous about maintaining twist to point of termination and, where possible, do maintain a seperation between power and data lines.
