Apparently this does happen, and it could happen to you. Right now, i've lost two of the locking knobs which fit the pan handle on the Manfrotto Video Heads 701HDV, and the one found on the 561BHDV-1 Video Monopod.
Luckily they do carry Manfrotto replacement knobs at B&H in case you happen to run into the same problem.
Replacement Pan Handle Locking Knob Manfrotto Video Head
If those little buggers are out of stock, you can always go for the entire Pan Handle part via Amazon (found here).
Manfrotto Replacement Pan Handle for 701HDV Video Fluid Head
