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
LT
Ebay item #262096601741
LT
You can also buy a M5 30mm knob on ebay for $1-2 instead of the overpriced Manfrotto replacement.
Vincent
Same thing happened back in November. I carry a little wrench with me the whole time now.
Emm
Post author@VanWeddings - Great to hear!
VanWeddings
@Emm - that tip worked great! thanks so much! I almost thought I needed to buy another monopod because of the problem but now it's good as new 😀
Emm
Post author@Adam - Thanks for pointing that out, corrected the link. I need a few of these.
Adam
actually the link you've provided is to the wrong part. that's the locking knob for the quick release plate (R701.212), i ordered it thinking it was for the arm, but it just ain't. the locking knob for the arm is R701.219
ttp://www.bhphotovideo.com/bnh/controller/home?O=&A=details&Q=&sku=777992&is=REG
Emm
Post author@VanWeddings - The tool works great, but make sure to unlock the clamp first. Tighten with the tool and then lock it again. I don't see why that wouldn't work.
VanWeddings
exact thing happened to my monopod. I still have the lever but the screw is missing. what I've found is that there are some advantages to just using the bare nut. for one, you don't have to worry about the lever turning into the bottom of the camera.
a bigger problem with my 561 monopod is that the top stage doesn't stay locked tight anymore. I've used the plastic tightening tool that came with the monopod, as well as tightened the metal screw, but it's still not tight enough. pushing hard on the top stage of the monopod will cause it to slowly collapse down. any ideas on how to fix that?
Kian
Now if you could find one that doesn't jut into my wrist when I am using the tripod arm parallel to my wrist.