Transmission

you actually need to use a real tracker not an example...
this usually means uploading a registering the torrent on a public site
or you can host your own tracker locally either using a client with an embedded tracker or using opentracker
also one of you will need to have port forwarding working in ...

i had this happen once (that i was aware of) when my hard drive was brand new
now about 2 months later i just noticed 3 torrents in a row get this

not sure how this happens, i assume either the data actually gets messed up after being written
or somehow the data is written badly after being cached ...