What is a hashfail on bittorrent?

Each torrent has a unique fingerprint called an info hash.

Among other information, the info hash has specific information about the content associated with the torrent.

As each "bit" of the content is downloaded, it is checked against the info hash to ensure it matches.

A "hashfail" is when content you have received through the torrent does not match the info hash's information on the content.

It is normal for a small amount of hashfails on a bittorrent transfer as the bits of the content being transferred sometimes get corrupted during the transfer.

These corrupted bits that fail the hash check are discarded and the bittorrent client looks for uncorrupted bits to replace them.

The hash check ensures that the final download will be uncorrupted.