If you routinely plug a USB flash drive into PCs in internet cafes, or even PCs where at some time untrusted people may have temporarily had access (even possibly your friend's or your own home PC), then at some point it is likely that a copy of your files will be covertly taken without your permission.

USB Hacksaw is one example of freely available software that makes this very easy to do:

http://wiki.hak5.org/wiki//USB_Hacksaw:

This web page gives a free download of the USB Hacksaw software. It takes a few minutes to download and install this to a USB flash drive. The rogue flash drive is then plugged into the USB port of a PC and automatically installs the USB Hacksaw software within seconds, with no need for user input. Here is a description of what happens next:

"This hack is based on a modified version of USBDumper. Once installed on a targert machine it will stay resident and wait for a USB flash drive to be inserted. Once a USB flash drive is inserted the hacksaw will download the contents of the drive to a temporary location using the modified USBDumper, then silently run the send.bat file located in the same directory, which will then archive the contents using RAR, eastablish an SSL SMTP connection to smtp.gmail.com using Stunnel and Blat, email the downloaded data to an email address, and remove the documents and archives."