Safari saves random files to Desktop in Mountain Lion
The .textClipping files are text that you've dragged out of an app to a folder location (in this case, the Desktop).

You can increase the delay required to drag a clipping using the following command:

defaults write -g NSDragAndDropTextDelay -int 10000


-g applies to all apps. To only apply to Safari, replace -g with com.apple.Safari.
Default delay is 1000 milliseconds (1 second).

To completely disable the ability to drag clippings, set the value to negative.

defaults write -g NSDragAndDropTextDelay -int -100