We've had a long term problem with amane returning a success code and content-length: 0 for perfectly valid files which are certainly longer than zero bytes. Once one of these zero byte files has been dished out the squids will happily hold on to it for a long time. Purging clears the problem for the impacted file.
The problem seems to go up and down in frequency depending on how overloaded amane is. It seems to be more frequent with larger files, but that might just be the biases of own experiences talking. People have toyed with the problem on and off but it's still not fixed.
Tim took the the OggHandler extension live today. The inline players have brought a lot more attention to media support.
But I've now gotten more bug reports related to stuck files than any other issue with the player. It's pretty miserable that our new feature is going to give a poor first impression to a lot of people because of this unresolved backend issue.