I've recently run into a frustrating problem that my short movie (only around 7 minutes in length total) will complete rendering successfully. Unfortunately when I attempt to watch the the video in VLC, movie player or other players the video appears to either crash or end around the 6 minute mark. I've tried different profiles such as HD PAL 720, NTSC, Web formats, etc. I've had the best luck with the HD PAL profile, and was actually able to render successfully once.
I'm currently using Ubuntu 12.04, pulling the latest Kdenlive and MLT from the PPA. I'm running on an Intel Sandy Bridge Core i7 CPU, with 4GB's of RAM. More than 100GB free of storage on my user partition.