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I have the same problem.
In one particular case, I mounted the remote machine under
/mnt/myth. After that I ran shfsumount /mnt/myth/ and got:
shfsumount: Device or resource busy

Then I ran (as root) lsof /mnt/myth/ and it returned

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
amarokapp 29569 testuser 17r DIR 0,18 512 2 /mnt/myth/

After running killall amarokapp, I get "/mnt/myth:
Input/output error" when running the "shfsumount /mnt/myth/"

seems that shfs can't whithstand abnormal exits....