Last week, one of my Raspberry Pi's locked up after it was physically moved while powered up. The symptoms showed up when I tried to connect to it through Remote Desktop. To troubleshoot and solve this, I hooked up a monitor to the HDMI port (through an HDMI-DVI converter) and tried rebooting. The Pi didn’t start. Since the SDHC card was rather stock, I reimaged it with Raspian Wheezy. Another reboot. This time it started, but the SDHC was corrupted, and showed sectors invalid. I tried a new card. It booted properly. Solved! (or so I thought). I installed xrdp, the remote management protocol and tried to reach the Pi from my laptop. It hung, after closer inspection I noticed that networking had dropped out, the message in syslog was "Link beat lost". During scanning of the forums I noticed how there seemed to be a general trend to blame this on the power supply. So I replaced it with a spare. Same problem. I then hooked up a volt meter to the 5V pin and GND. When running, but more or less idling, it showed roughly 4.65 volts. When I tried to start Remote Desktop, it dropped to 4.55 volts and then networking failed.
At long last, I decided to swap power supply cables. Problem solved. DVM showed steady voltage level of 5.05 volts. Remote Desktop started properly and remained connected for the duration of the session.
I bought a bunch of micro USB cables on eBay a number of weeks ago. The faulty cable was one of them.
The moral of the story: not everything on eBay is naturally a bargain.