I've getting problems copying files from one Windows 2012 server to another via Cisco ASA5505 VPN. I've had a similar setup with Win2008 servers & never saw the problem (same ASA5505 this end)
For about 1 in every 20 files (once or twice a day) of about 20MB or more, I tend to get data corruption.
The corruption tends to be in the form of about 5 areas of corruption, (sometimes all localised within a 1MB section of the file), and each area is about 12 bytes long, totalling in approx 60 corrupt bytes in the whole file. The file size does not change.
I've seen this with the native SQLServer log-ship-copy and with robocopy.
Connected with this is that when I use windows drag/drop to copy a 40GB file, the copy usually fails around after 15-30GB (or copies with some corruption). This doesn't seem to happen when I copy Win2012 -> VPN -> SBS2011, then copy SBS2011 -> Win2012.
Compression does not appear to be enabled on the ASA5505. No errors spotted on a the standard NetMonitor SMBErrors trace.