Seems that smbmount does not restore the connection after a failure.
To reproduce do the following (the ssh is used only to demonstrate the 
problem, it is irrevant for the problem).

1. Open a local tunnel to server host
ssh -L 8001:localhost:139 user.com
a tunnell local port (8001)  will be open.

2. mount a disk using smbmount via this tunnel.
smbmount //127.0.0.1/share_name /mnt -o username=samba_user_name,port=8001
Now you can browse the mounted filesystem
ls -l /mnt

3. Close ssh session (do not forget to close the terminal, because
otherwise
the ssh will continue to provide tunnell)
so the tunnell will not be available.
Do ls -l /mnt
you will get "IO error" message as it should be.

4. Re-establish ssh session (repeat item 1)
doing ls -l /mnt you receive "IO error"
despite the connection was re-established. 
Buy the way Windows NT reestablishes the connection automatically in such
cases.

The only way to fix "IO error" this is to
umount /mnt and then mount it again (item #2)