Some demos refuse to play for me...

I just downloaded some demos from gotfrag, to have a look at how the pros play, and maybe one third of them work. The other two-thirds stop loading 3/4 of the way through and crash back to the main menu. Other people don't have this problem, just me.

Most of those demos are from earlier releases of TF2, and the updates since have broken them. The console will complain that the demo doesn't contain information that matches what the current version is used to, and won't play it. There is a guide (here) for patching a separate installation of the game to an older version, so it can play old demos. And you are most certainly not the only one with this issue. Most of my dominant performances have been lost due to incompatibility, but this patch makes some of them work.