Why do some streamed FLV's not play in Media Player Classic?

You should look into using a different capturing method.

Here's an example of one not working: (201.16MB) It looks like they're appending an intro to the video that is messing it up in MPC-HC's splitter. It works just fine in VLC and mplayer. By the way, this is part 4 of a 4 part podcast called The Bonus Round on GameTrailers.com. If you download all of the episode from the start there's no problem. The problems only happen when you just want to download one part of the episode that is not part 1.

clsid: I find capturing streams kind of tricky... I've downloaded like 5 programs that claim to capture screens directly (rather than screencap) and I'll be lucky if 1 of them works. Do you have any advice on alternate capturing methods? Two different programs I've used seem to rely on winpcap. In this case, I'm not even sure if it should be called a stream... though it came from ustream.tv. It's a saved broadcast and has a permanent(?) flv link: Mine doesn't seem to have any intro so I don't know if my issue is the same as Schrade's. Can I use a different splitter? I tried using MPC's native decoder instead of ffdshow and that didn't work, is there one more I should try?

creedo, that played just fine for me. Although I did open it via the URL and streamed it in MPC-HC. I didn't download the file fully to play it. Am downloading it now to check it out. I'm using MPC-HC v1.3.1771.0 btw. I downloaded the entire file and again, it played just fine for me.

I use the DownloadHelper plugin for Firefox. That normally works great. I tried it on your GameTrailer example to get just the 4th part and it also resulted in a file that had playback problems. The FLV splitter does not like the file. The during is wrong, which also messes up the seeking. Moitah has made some fixes for the FLV splitter in the past. You could ask him to take a look at this issue. Maybe he is willing to help.