mount error(12): Cannot allocate memory
Refer to the mount.cifs(8) manual page (e.g.man mount.cifs)

After checking the Event Viewer System log, I found the following error:

Source: srv
Event ID: 2017
Level: Error
The server was unable to allocate from the system nonpaged pool because the server reached the configured limit for nonpaged pool allocations.

Some research led me to find this Google Groups discussion about the problem and this Microsoft Technet article discussing the solution (look at the bottom of the page).  Apparently you need to tell Windows that you want to use the machine as a file server and that it should allocate resources accordingly.  Set the following registry key to ’1′:

HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\LargeSystemCache

and set the following registry key to ’3′:

HKLM\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters\Size

After making these changes and restarting, I haven’t seen this issue arise again.  Fixed!

Update (2011/9/16): I had no idea this fix would end up being so popular when I wrote it up.  It is easily the most visited page on this site.  The original Technet article seems to be gone now, but a commenter has pointed me to a text file that contains the same content.  Here is the link: http://www.decuslib.com/decus/vmslt99a/nt/tips.txt