I happened to stumble upon post #494975. The post is marked as active, but the source url and sample url return a 404 error. (The preview url works fine, though.)
This was touched upon by a comment about a month ago, but never followed up on.

How do we bring that to attention? The report link doesn't cover that. I don't want to flag it for deletion (though I have seen "broken/corrupted file" in FFD reasons) but rather have it fixed (if possible).
I might find more

And on a side-note: How did this happen?

Flag the post for being broken/corrupted.

This sometimes happen and flagging it pokes us to fix it or delete it. There was actually Zonkpunch flash file I replaced yesterday, because it was downloading data from 404 source, it was essentially useless and artist did have offline version available in couple sources, but if there wasn't, the flash would've been pretty much useless to keep around anyway when you couldn't watch it.

Someone doing the code can maybe explain better why it happens, all I know it can happen and how to fix it.

Ok, thank you for the explanation.
Will do that.