This happens when a keypress is passed to a client but the key release was never passed. Caused by having your focus on one client, pressing a key, then changing the focus to a different client before you release the key. The other way this can happen is with modifiers. If you have <ctrl> w as your slaves forward key you need to be consious of the order you press them.
If you press ctrl, then w then release w then release ctrl, your slaves will see this:
press <ctrl> w
release <ctrl> w

how ever, if you press ctrl, then press w, then release ctrl then release w your slaves will see this
press <ctrl> w
release w

Thus they have not seen the command to release <ctrl> w and are still going to be moving. So what it comes down to is that you need to train yourself to be very consious of your key presses and orders and to which window you have focus. As mikekim said "the spinning around thing is possibly due to having focus follows mouse enabled on keyclone - this is a known issue." This is another thing to keep an eye on for the same reason, the window changes focus and the client never recieves the release command.