I drop things and sometimes have trouble walking. In my case I've finally figured out it is caused by a single, quick muscle spasm in my arm or leg. Each spasm is so quick, it took me a long time to recognize what it was. At first I thought it was momentary weakness.
It's most obvious when I am walking and my knee suddenly starts to collapse, then I recover. I kind of looks like I stepped off a curb for a moment, but I'm walking on flat ground.
The spasms in my hands seem to affect my thumb the most. If what I'm holding is resting against my thumb, I drop it.
At work I've taken to carrying things with my palms or fingers when possible to minimize the odds of dropping anything. Sometimes I have to use my thumbs though. It just drive me nuts - some days I seem to get a spasm every 5 minutes. It usually doesn't last for the whole day, though.
I saw a neurologist. He thinks I am reacting to the serotonin uptake inhibitors I am taking for fibro. I'm not sure I agree because I am on pretty low doses, but I am not willing to try to disprove his theory (I would have to stop taking some of my medications). If it is not the medications, then it is idiopathic, meaning there is no underlying cause for the symptom.