My most recent one was a Teltone telephone line simulator, specifically their model TLE-A-01. Pretty neat device... It can emulate a central office, including sending caller ID data, which makes it ideal for development and test work on an Asterisk PBX.
Anyway... These units have a VF display on the front panel, connected to the mainboard by a flat cable. The one I had was dead, to initial appearances, so I got it for cheap (I thought its power supply might have gone wonkers).
What did I find when I got it on the bench and opened it up? The flat cable for the display had come disconnected. It took less than a minute to reconnect and verify the unit was just fine.
'Tis truly amazing how little effort some folks are willing to put out when something doesn't instantly respond as expected the first time.