I just had this problem a minute ago.

The viewport starting moving forward constantly, as if I were holding down the w key. It wouldn't stop, unless I held down the s key, which would bring the viewport to a standstill.

Just prior to the bug occuring I was in move mode, dragging something across the xy axes in view space. I think I also hit d to translate the viewport right (so I could follow what I was moving). As soon as I released the d key, and also the mouse1 (since I'd finished my move operation) the viewport started moving.

I rebooted Sandbox, no effect.
I loaded different levels in Sandbox, no effect.
I unplugged the keyboard, no effect.
I looked for help on google, which brought me back here saying to double tap s, no effect.
Eventually I mashed all the keys, no effect.
I started tapping s and the down key repeatedly, it stopped doing it.

Anyone else had this?