QUOTE(Anto of the Sand @ Nov 20 2011, 13:20)

Okay, my VLC worked fine, but this issue can be sporadic. Any ideas what it might be?
It might be, for example, specific video codec used in the video you play, when a specific algorithm used by video player tries to decode its data.
Files like AVI, MKV, OGM are just containers. They contain video and audio data (possibly more than one in case of OGM and MKV, and subtitle texts too). However, the video data can be encoded with one of many video codecs like these: [
en.wikipedia.org]
http://en.wikipedia.org/wiki/Video_codec , and not all players can decode all codecs equally well.
Since this started happening three weeks ago, possibly you had a software update to your video player around that time, which introduced a new bug showing up when trying to decode data for some specific codec, or more codecs?