Could be an indication of a relatively slow hard drive that is just not delivering data fast enough. Most likely to occur when one or more recordings are taking place simultaneously with playback or a previously recorded program, thus really putting the screws on the hard drive performance specs.
I had to offload quite a bit of Summer Olympics recordings from my high-speed large-cache internal SATA drives (primary recording folder for WMC) to make room for the hundreds of hours of incoming new recordings over the 16 days, and my 2TB external USB 3.0 drive (Verbatim, which is actually a Samsung HD204UI SATA drive inside) seemed like the right place for it. Well, space became an issue even on that drive, so I ran out and bought a second 2TB USB 3.0 drive (this time a HitachiGST SATA drive inside).
So I have recordings split across the two drives. Both are supported by a USB 3.0 PCIe adapter in the PC.
I never see any of these "media file errors" coming from playback of the recordings on the Verbatim/Samsung 2TB drive, but see them fairly often (and annoyingly) playing recordings from the Hitachi 2TB drive.
I can't recall seeing these errors ever when playing recordings from internal SATA-II hard drive folders in the past. With my motherboard upgrade (to ASUS P8Z77-V Pro) I've now upgraded my internal hard drives to 2TB SATA-III as well, which are even faster and have even larger cache... and I NEVER see these errors.
==> "Green" drives may run cooler and quieter and use less electricity, but they also do not have the performance required if you're going to be recording 3-4 programs at once while also watching programs (live or recorded, doesn't matter). Remember that watching "live" actually goes through the hard drive just as recording a program does, to accomplish the 30-minute buffering in support of "pause live", "rewind live", etc.
I've opted for high-performance SATA drives, since high reliability error-free WMC record/playback is one of my uncompromising criteria.