I've often returned to PuF to find the sigs broken and I cannot figure out where the error occurs.
There's no error. That's a specific feature they employ to prevent what's known as "direct linking", which is when people attempt to use these site's hard drive space as a free ftp server for files.
These places are there to make money. They do so by giving you some web space and hope that you have enough interesting stuff that at least a few people will come visit it, thereby getting a few more eyeballs on their ads, and money from the advertisers.
However, if you are just having files directly linked from another page, this circumvents their business plan, and you've just used their bandwidth, hard drive space and so forth, without giving anything in return.
It's simple economics, and why more of these "free" webspace sites will continue to close down.
